Use FrameTreeNode ID as frameId in extension APIs

content/public/browser/web_contents.h

Index: content/public/browser/web_contents.h
diff --git a/content/public/browser/web_contents.h b/content/public/browser/web_contents.h
index 6f68c96273a453be2afb952657410e8c5908a2e5..bc13dc97accc67b357a17e8df2744a5cbac31d23 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -214,6 +214,12 @@ class WebContents : public PageNavigator,
   // Returns the focused frame for the currently active view.
   virtual RenderFrameHost* GetFocusedFrame() = 0;
 
+  // Returns the current RenderFrameHost for a given FrameTreeNode ID if it is
+  // part of this tab. See RenderFrameHost::GetFrameTreeNodeId for documentation
+  // on this ID.
+  virtual RenderFrameHost* FindFrameByFrameTreeNodeId(
+      int frame_tree_node_id) = 0;
+
   // Calls |on_frame| for each frame in the currently active view.
   // Note: The RenderFrameHost parameter is not guaranteed to have a live
   // RenderFrame counterpart in the renderer process. Callbacks should check