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 7e769cb154ad7754de70a7559025c733556dfd8f..26853765c8649e26c9a7ca835407ca53f5ad949f 100644
--- a/content/public/browser/render_frame_host.h
+++ b/content/public/browser/render_frame_host.h
@@ -35,6 +35,10 @@ 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 hosts the frame identified by the given
+  // FrameTreeNode ID. Returns nullptr if not found.
+  static RenderFrameHost* FromFrameTreeNodeID(int frame_tree_node_id);
+
   // 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
   // configurations.
@@ -64,6 +68,13 @@ class CONTENT_EXPORT RenderFrameHost : public IPC::Listener,
   // current RenderFrameHost.
   virtual RenderFrameHost* GetParent() = 0;
 
+  // Returns the RenderFrameHost for the given FrameTreeNode ID if it is the

[Charlie Reis, 2015/10/30 23:25:09]

nit: the current RenderFrameHost
(The same FrameTreeNode ID can refer to different RenderFrameHosts at different
times, due to cross-process navigations.)

[robwu, 2015/11/02 19:08:34]

Done.

+  // frame itself or a child of this frame.

[Charlie Reis, 2015/10/30 23:25:09]

nit: or an immediate child

(Is this what you intended?  I'm not sure about the use cases.  If you just need
to find any frame in a given FrameTree, the method might belong in WebContents
instead.)

[robwu, 2015/10/31 00:10:45]

My use case is finding a frame within a given tab, so moving to a WebContents
sounds good.

[robwu, 2015/11/02 19:08:34]

Done.

+  virtual RenderFrameHost* FindByFrameTreeNodeID(int frame_tree_node_id) = 0;
+
+  // Returns the FrameTreeNode ID for this frame.
+  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