Use FrameTreeNode ID as frameId in extension APIs

chrome/common/extensions/docs/templates/intros/webNavigation.html

 <h2 id="manifest">Manifest</h2>
 <p>
 All <code>chrome.webNavigation</code> methods and events require you to declare
 the "webNavigation" permission in the <a href="manifest">extension
 manifest</a>.
 For example:
 </p>
 
 <pre data-filename="manifest.json">
 {
@@ skipping 85 matching lines @@
 It's important to note that some technical oddities in the OS's handling
 of distinct Chrome processes can cause the clock to be skewed between the
 browser itself and extension processes. That means that WebNavigation's events'
 <code>timeStamp</code> property is only guaranteed to be <i>internally</i>
 consistent. Comparing one event to another event will give you the correct
 offset between them, but comparing them to the current time inside the
 extension (via <code>(new Date()).getTime()</code>, for instance) might give
 unexpected results.
 </p>
 
-<h2 id="frame_ids">A note about frame and process IDs</h2>
+<h2 id="frame_ids">A note about frame IDs</h2>
 <p>
 Frames within a tab can be identified by a frame ID. The frame ID of the main
 frame is always 0, the ID of child frames is a positive number. Once a document
 is constructed in a frame, its frame ID remains constant during the lifetime of
-the document.
+the document. As of Chrome 48, this ID is also constant for the lifetime of the

[Devlin, 2015/12/10 21:53:06]

Chrome 49?

[robwu, 2015/12/11 00:53:47]

Done.

+frame (across multiple navigations).
 </p>
 <p>
 Due to the multi-process nature of Chrome, a tab might use different processes
 to render the source and destination of a web page. Therefore, if a navigation
 takes place in a new process, you might receive events both from the new and
 the old page until the new navigation is committed (i.e. the
-<code>onCommitted</code> event is send for the new main frame). Because frame
-IDs are only unique for a given process, the webNavigation events include a
-process ID, so you can still determine which frame a navigation came from.
+<code>onCommitted</code> event is send for the new main frame). In other words,
+it is possible to have more than one pending sequence of webNavigation events
+with the same <code>frameId</code>. The sequences can be distinguished by the
+<code>processId</code> key.
 </p>
 <p>
 Also note that during a provisional load the process might be switched several
 times. This happens when the load is redirected to a different site. In this
 case, you will receive repeated <code>onBeforeNavigate</code> and
 <code>onErrorOccurred</code> events, until you receive the final
 <code>onCommitted</code> event.
 </p>
 
 <h2 id="transition_types">Transition types and qualifiers</h2>
@@ skipping 32 matching lines @@
     The user used the Forward or Back button to initiate the navigation.
   </td>
 </tr>
 <tr>
   <td>"from_address_bar"</td>
   <td>
     The user initiated the navigation from the address bar (aka Omnibox).
   </td>
 </tr>
 </table>