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..071ad636f9c4777c24ec7e7fb7bf19eaba880493 100644
--- a/content/public/browser/render_frame_host.h
+++ b/content/public/browser/render_frame_host.h
@@ -64,6 +64,9 @@ class CONTENT_EXPORT RenderFrameHost : public IPC::Listener,
   // current RenderFrameHost.
   virtual RenderFrameHost* GetParent() = 0;
 
+  // Returns the FrameTreeNode ID for this frame.

[Charlie Reis, 2015/11/06 23:13:16]

Nasko brings up a good point that we may not want to call this
"GetFrameTreeNodeID," since there's no FrameTreeNode in the public API... 
Sadly, "frame ID" is slightly ambiguous since we already have render_frame_id in
some APIs (which confusingly refers to RenderFrame routing ID).

I'm not sure if there's a better option-- GetFrameID (and bite the bullet vs
render_frame_id) vs somehow explaining what GetFrameTreeNodeID means vs
something else?

Either way, we should elaborate on what this concept means here, since there's
no other explanation in the public API.  In other words, that it is a stable
identifier for a frame even after it does cross-process navigations that switch
its RenderFrameHost.

+  virtual int GetFrameTreeNodeID() = 0;

[nasko, 2015/11/06 23:28:27]

Let's call this GetFrameId(). FrameTreeNode is not a concept exposed outside of
content/ and shouldn't be leaked, even through naming of methods.
Also, include another sentence which describes the meaning of this id - a unique
ID for this frame, which will not change even across cross-process navigations.

[robwu, 2015/11/07 00:23:40]

It is very likely that GetFrameId is confused with the RenderFrame routing ID,
so we need a name that is a bit more specific than GetFrameId.

+
   // 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