Correct PushMessageData

Give PushMessageData a clear internal concept to hold the bytes and also make it clear that it cannot be null. None of the methods were prepared to handle it being null and nothing ever creates a PushMessageData object where it is null.

Partially addresses the concerns in #380 and fixes #381.

index.html

@@ -982,30 +982,29 @@ <h2>
         };
       </pre>
       <p>
-        <a>PushMessageData</a> objects have an associated [=byte sequence=] set on creation, which
-        is `null` if there was no data in the <a>push message</a>.
+        <a>PushMessageData</a> objects have an associated <dfn for=PushMessageData>bytes</dfn> (a
+        [=byte sequence=]), which is set on creation.
       </p>
       <p>
-        The <dfn>arrayBuffer()</dfn> method, when invoked, MUST return an {{ArrayBuffer}} whose
-        contents are |bytes|. Exceptions thrown during the creation of the {{ArrayBuffer}} object
-        are re-thrown.
+        The <dfn>arrayBuffer()</dfn> method steps are to return an {{ArrayBuffer}} whose contents
+        are <a>this</a>'s <a for=PushMessageData>bytes</a>. Exceptions thrown during the creation of
+        the {{ArrayBuffer}} object are re-thrown.
       </p>
       <p>
-        The <dfn>blob()</dfn> method, when invoked, MUST return a {{Blob}} whose contents are
-        |bytes| and |type| is not provided.
+        The <dfn>blob()</dfn> method steps are to return a new {{Blob}} object whose contents are
+        <a>this</a>'s <a for=PushMessageData>bytes</a>.
       </p>
       <p>
-        The <dfn>bytes()</dfn> method, when invoked, MUST return a {{Uint8Array}} backed by a
-        {{ArrayBuffer}} whose contents are |bytes|. Exceptions thrown during the creation of the
+        The <dfn>bytes()</dfn> method steps are to return a new {{Uint8Array}} backed by a
+        {{ArrayBuffer}} whose contents are <a>this</a>'s <a for=PushMessageData>bytes</a>. Exceptions thrown during the creation of the
         {{ArrayBuffer}} object are re-thrown.
       </p>
       <p data-cite="encoding">
-        The <dfn>json()</dfn> method, when invoked, MUST return the result of invoking the initial
-        value of `JSON`.{{JSON/parse()}} with the result of running <a>utf-8 decode</a> on |bytes|
-        as argument. Re-throw any exceptions thrown by `JSON`.{{JSON/parse()}}.
+        The <dfn>json()</dfn> method steps are to return the result of
+        <a>parsing JSON bytes to a JavaScript value</a> given <a>this</a>'s <a for=PushMessageData>bytes</a>.
       </p>
       <p data-cite="encoding">
-        The <dfn>text</dfn> method, when invoked, MUST return the result of running <a>utf-8
+        The <dfn>text</dfn> method, when invoked, MUST return the result of running <a>UTF-8
         decode</a> on |bytes|.
       </p>
       <p>