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

content/browser/frame_host/render_frame_host_delegate.h

Index: content/browser/frame_host/render_frame_host_delegate.h
diff --git a/content/browser/frame_host/render_frame_host_delegate.h b/content/browser/frame_host/render_frame_host_delegate.h
index 120e18ed8357f4fdc6a55d8a2cd7b884c29404d3..87a089d2dee59ca10b2eb338dce5fc03dc08a8db 100644
--- a/content/browser/frame_host/render_frame_host_delegate.h
+++ b/content/browser/frame_host/render_frame_host_delegate.h
@@ -231,6 +231,10 @@ class CONTENT_EXPORT RenderFrameHostDelegate {
   // 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(