Use FrameTreeNode ID as frameId in extension APIs

content/public/browser/render_frame_host.h

Index: content/public/browser/render_frame_host.h
diff --git a/content/public/browser/render_frame_host.h b/content/public/browser/render_frame_host.h
index 0fcb180954337def1462781c229008164f893aa7..6d5e9ae2dd505a8a5d22744759db9c1916b5ccb2 100644
--- a/content/public/browser/render_frame_host.h
+++ b/content/public/browser/render_frame_host.h
@@ -36,6 +36,15 @@ class CONTENT_EXPORT RenderFrameHost : public IPC::Listener,
   // Returns nullptr if the IDs do not correspond to a live RenderFrameHost.
   static RenderFrameHost* FromID(int render_process_id, int render_frame_id);
 
+  // Returns the RenderFrameHost that is hosted by the frame identified by the

[nasko, 2015/12/09 23:06:09]

nit: s/hosted/associated/

[robwu, 2015/12/10 00:00:51]

Done.

+  // given FrameTreeNode ID. The returned RenderFrameHost can be different from
+  // the RenderFrameHost that returned the ID via GetFrameTreeNodeID(); see the
+  // documentation at GetFrameTreeNodeID for more details.
+  // Use WebContents::FindFrameByFrameTreeNodeID to find a RenderFrameHost in
+  // a specific WebContents.
+  // Returns nullptr if the frame does not exist (any more).

[nasko, 2015/12/09 23:06:09]

nit: You can drop the "(any more)".

[robwu, 2015/12/10 00:00:51]

Done.

+  static RenderFrameHost* FromFrameTreeNodeID(int frame_tree_node_id);

[nasko, 2015/12/09 23:06:09]

The conclusion on capitalization is using Id instead of ID.

[robwu, 2015/12/10 00:00:51]

Done.

+
 #if defined(OS_ANDROID)
   // Globally allows for injecting JavaScript into the main world. This feature
   // is present only to support Android WebView and must not be used in other
@@ -67,6 +76,17 @@ class CONTENT_EXPORT RenderFrameHost : public IPC::Listener,
   // current RenderFrameHost.
   virtual RenderFrameHost* GetParent() = 0;
 
+  // Returns the FrameTreeNode ID for this frame. This ID is browser-global and
+  // uniquely identifies a frame (including main frames) that hosts content. The

[nasko, 2015/12/09 23:06:09]

nit: Drop the "(including main frames)" as this is a bit of context specific to
the extension APIs.

[robwu, 2015/12/10 00:00:51]

Done.

+  // identifier is fixed at the creation of the frame and stays constant for the
+  // lifetime of the frame. When the frame is removed, the ID is not used again.
+  //
+  // A RenderFrameHost is tied to a process. Due to cross-process navigations,
+  // the RenderFrameHost may have a shorter lifetime than a frame. Consequently,
+  // the FrameTreeNode ID may refer to a different RenderFrameHost after a
+  // navigation.
+  virtual int GetFrameTreeNodeID() = 0;
+
   // Returns the assigned name of the frame, the name of the iframe tag
   // declaring it. For example, <iframe name="framename">[...]</iframe>. It is
   // quite possible for a frame to have no name, in which case GetFrameName will