[Conditional Focus] Add CaptureController to getDisplayMedia options

This CL adds a new CaptureController object that can be passed to
navigator.mediaDevices.getDisplayMedia() so that web developers
can control whether the captured surface (tab or window only) will be
focused or not.

Spec: w3c/mediacapture-screen-share#230
Demo: https://capture-controller.glitch.me/
Bug: 1215480
Change-Id: Ib826b702fc853542a0c5bae5ddac3286216e2d63

screen-capture/getdisplaymedia-capture-controller.https.window.js

@@ -0,0 +1,42 @@
+// META: script=/resources/testharness.js
+// META: script=/resources/testharnessreport.js
+// META: script=/resources/testdriver.js
+// META: script=/resources/testdriver-vendor.js
+
+'use strict';
+
+test(() => {
+  assert_own_property(window, 'CaptureController');
+}, 'CaptureController in window');
+
+const stopTracks = stream => stream.getTracks().forEach(track => track.stop());
+
+promise_test(async t => {
+  const controller = new CaptureController();
+  await test_driver.bless('getDisplayMedia()');
+  const stream = await navigator.mediaDevices.getDisplayMedia({controller});
+  t.add_cleanup(() => stopTracks(stream));
+  assert_equals(stream.getTracks().length, 1);
+  assert_equals(stream.getVideoTracks().length, 1);
+  assert_equals(stream.getAudioTracks().length, 0);
+}, 'getDisplayMedia(controller) must succeed');
+
+promise_test(async t => {
+  const controller = new CaptureController();
+
+  await test_driver.bless('getDisplayMedia()');
+  const stream = await navigator.mediaDevices.getDisplayMedia({controller});
+  t.add_cleanup(() => stopTracks(stream));
+
+  await test_driver.bless('getDisplayMedia()');
+  const p = navigator.mediaDevices.getDisplayMedia({controller});
+  t.add_cleanup(async () => {
+    try {
+      stopTracks(await p)
+    } catch {
+    }
+  });
+  await promise_rejects_dom(
+      t, 'InvalidStateError', Promise.race([p, Promise.resolve()]),
+      'getDisplayMedia should have returned an already-rejected promise.');
+}, 'getDisplayMedia(controller) must fail with InvalidStateError if controller is bound');