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

content/browser/web_contents/web_contents_impl.h

 // Copyright (c) 2012 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_WEB_CONTENTS_WEB_CONTENTS_IMPL_H_
 #define CONTENT_BROWSER_WEB_CONTENTS_WEB_CONTENTS_IMPL_H_
 
 #include <stdint.h>
 
 #include <functional>
@@ skipping 498 matching lines @@
   device::mojom::WakeLockContext* GetWakeLockServiceContext() override;
   void EnterFullscreenMode(const GURL& origin) override;
   void ExitFullscreenMode(bool will_cause_resize) override;
   bool ShouldRouteMessageEvent(
       RenderFrameHost* target_rfh,
       SiteInstance* source_site_instance) const override;
   void EnsureOpenerProxiesExist(RenderFrameHost* source_rfh) override;
   std::unique_ptr<WebUIImpl> CreateWebUIForRenderFrameHost(
       const GURL& url) override;
   void SetFocusedFrame(FrameTreeNode* node, SiteInstance* source) override;
+  void DidChangeOpener(FrameTreeNode* node, FrameTreeNode* opener) override;
   void OnFocusedElementChangedInFrame(
       RenderFrameHostImpl* frame,
       const gfx::Rect& bounds_in_root_view) override;
   void CreateNewWindow(
       RenderFrameHost* opener,
       int32_t render_view_route_id,
       int32_t main_frame_route_id,
       int32_t main_frame_widget_route_id,
       const mojom::CreateNewWindowParams& params,
       SessionStorageNamespace* session_storage_namespace) override;
@@ skipping 1061 matching lines @@
   // Adds/removes a callback called on creation of each new WebContents.
   static void AddCreatedCallbackForTesting(const CreatedCallback& callback);
   static void RemoveCreatedCallbackForTesting(const CreatedCallback& callback);
 
   DISALLOW_COPY_AND_ASSIGN(FriendWrapper);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_WEB_CONTENTS_WEB_CONTENTS_IMPL_H_