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

chrome/browser/background/background_contents.h

Index: chrome/browser/background/background_contents.h
diff --git a/chrome/browser/background/background_contents.h b/chrome/browser/background/background_contents.h
index f5e28039a20fca0817fcd371e5968b75cd72d2b1..0708f1c50297185719c58e6b0f081eda9ecbd704 100644
--- a/chrome/browser/background/background_contents.h
+++ b/chrome/browser/background/background_contents.h
@@ -24,6 +24,7 @@
 class Profile;
 
 namespace content {
+class RenderFrameHost;
 class SessionStorageNamespace;
 };
 
@@ -121,6 +122,15 @@ class BackgroundContents : public extensions::DeferredStartRenderHost,
   // The initial URL to load.
   GURL initial_url_;
 
+  // TODO(lukasza): Remove members below after gathering sufficient UMA data for
+  // https://crbug.com/721947.
+  void DidChangeOpener() override;
+  void TrackPreviousOpener();
+  void OnPreviousOpenerDestroyed();
+  bool IsCurrentOpenerHostedApp();
+  content::RenderFrameHost* previous_opener_ = nullptr;
+  base::WeakPtrFactory<BackgroundContents> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(BackgroundContents);
 };