Web Share: Only allow one share() call at a time

Spec:
https://w3c.github.io/web-share/#share-method

If a share() request is active when share() is called again, the new
share request fails immediately.
w3c/web-share#113

In content shell, the OS-integration for the share service is not
present. Instead of crashing in tests, we report a not implemented
error.

WPT web-share/share-sharePromise-internal-slot.https.html now passes -
it previously crashed.
w3c/web-share#183 improves consistency between
the test and the spec.

Protocols other that http and https are no longer supported by
share() or canShare().
w3c/web-share#174

canShare is being discussed in
w3c/web-share#177

Bug: 1002337, 1002514, 1131755
Change-Id: I4ec9f6eb03373fd5c6db1881df906a8df36ca4ff

web-share/canShare.tentative.https.html

@@ -11,74 +11,74 @@
       'use strict';
 
       test(() => {
-        assert_equals(navigator.canShare(), false);
+        assert_false(navigator.canShare());
       }, 'canShare with no arguments (same as empty dictionary)');
 
       test(() => {
-        assert_equals(navigator.canShare({}), false);
+        assert_false(navigator.canShare({}));
       }, 'canShare with an empty dictionary');
 
       test(() => {
-        assert_equals(navigator.canShare(undefined), false);
+        assert_false(navigator.canShare(undefined));
       }, 'canShare with a undefined argument (same as empty dictionary)');
 
       test(() => {
-        assert_equals(navigator.canShare(null), false);
+        assert_false(navigator.canShare(null));
       }, 'canShare with a null argument (same as empty dictionary)');
 
       test(() => {
-        assert_equals(navigator.canShare({unused: 'unexpected field'}), false);
+        assert_false(navigator.canShare({unused: 'unexpected field'}));
       }, 'canShare with a dictionary containing only surplus fields');
 
       test(() => {
         // URL is invalid in that the URL Parser returns failure (port is too
         // large).
         const url = 'http://example.com:65536';
-        assert_equals(navigator.canShare({url}), false);
+        assert_false(navigator.canShare({url}));
       }, 'canShare with an invalid URL');
 
       test(() => {
-        assert_equals(navigator.canShare({title: undefined}), false);
+        assert_false(navigator.canShare({title: undefined}));
       }, 'canShare with attribute undefined is equivalent to omitting the attribute');
 
       test(() => {
-        assert_equals(navigator.canShare({title: 'subject'}), true);
+        assert_true(navigator.canShare({title: 'subject'}));
       }, 'canShare with title');
 
       test(() => {
-        assert_equals(navigator.canShare({text: 'body'}), true);
+        assert_true(navigator.canShare({text: 'body'}));
       }, 'canShare with text');
 
       test(() => {
-        assert_equals(navigator.canShare({url: 'https://www.example.com/some/path?some_query#some_fragment'}), true);
+        assert_true(navigator.canShare({url: 'https://www.example.com/some/path?some_query#some_fragment'}));
       }, 'canShare with URL');
 
       test(() => {
-        assert_equals(navigator.canShare({title: null}), true);
+        assert_true(navigator.canShare({title: null}));
       }, 'canShare with null attribute');
 
       test(() => {
-        assert_equals(navigator.canShare({text: 123}), true);
+        assert_true(navigator.canShare({text: 123}));
       }, 'canShare with number');
 
       test(() => {
-        assert_equals(navigator.canShare({url: {toString() { return 'https://example.com/'; }}}), true);
+        assert_true(navigator.canShare({url: {toString() { return 'https://example.com/'; }}}));
       }, 'canShare with object');
 
       test(() => {
-        assert_equals(navigator.canShare({title: 'subject', text: 'body', url: 'https://example.com/', unused: 'unexpected field'}), true);
+        assert_true(navigator.canShare({title: 'subject', text: 'body', url: 'https://example.com/', unused: 'unexpected field'}));
       }, 'canShare with unexpected field');
 
       test(() => {
-        assert_equals(navigator.canShare({url: 'data:the url'}), true);
+        assert_false(navigator.canShare({url: 'data:the url'}));
       }, 'canShare with data URL');
 
       test(() => {
-        assert_equals(navigator.canShare({url: ''}), true);
+        assert_true(navigator.canShare({url: ''}));
       }, 'canShare with empty URL');
 
       test(() => {
-        assert_equals(navigator.canShare({url: '//www.example.com/some/path?some_query#some_fragment'}), true);
+        assert_true(navigator.canShare({url: '//www.example.com/some/path?some_query#some_fragment'}));
       }, 'canShare with URL having no scheme');
     </script>
   </body>