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 c9fa8a011e04fed5db3aad0bb80354a194b29158..2818f3ff7b820bcf518cc2e09082340e62bc2aac 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -212,6 +212,12 @@ 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