UMA for measuring how often 2 or more tabs script the same background contents.

content/browser/frame_host/render_frame_host_delegate.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_DELEGATE_H_
 #define CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_DELEGATE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 213 matching lines @@
   // special case for <webview> guests, but this logic should eventually be
   // moved down into RenderFrameProxyHost::RouteMessageEvent when <webview>
   // refactoring for --site-per-process mode is further along.  See
   // https://crbug.com/330264.
   virtual void EnsureOpenerProxiesExist(RenderFrameHost* source_rfh) {}
 
   // Set the |node| frame as focused in the current FrameTree as well as
   // possibly changing focus in distinct but related inner/outer WebContents.
   virtual void SetFocusedFrame(FrameTreeNode* node, SiteInstance* source) {}
 
+  // Set the |node| frame as focused in the current FrameTree as well as
+  // possibly changing focus in distinct but related inner/outer WebContents.
+  virtual void DidChangeOpener(FrameTreeNode* node, FrameTreeNode* opener) {}
+
   // Creates a WebUI object for a frame navigating to |url|. If no WebUI
   // applies, returns null.
   virtual std::unique_ptr<WebUIImpl> CreateWebUIForRenderFrameHost(
       const GURL& url);
 
   // Called by |frame| to notify that it has received an update on focused
   // element. |bounds_in_root_view| is the rectangle containing the element that
   // is focused and is with respect to root frame's RenderWidgetHost's
   // coordinate space.
   virtual void OnFocusedElementChangedInFrame(
@@ skipping 60 matching lines @@
   GetJavaRenderFrameHostDelegate();
 #endif
 
  protected:
   virtual ~RenderFrameHostDelegate() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_DELEGATE_H_