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

content/browser/web_contents/web_contents_impl.cc

 // 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.
 
 #include "content/browser/web_contents/web_contents_impl.h"
 
 #include <stddef.h>
 
 #include <cmath>
 #include <utility>
@@ skipping 4910 matching lines @@
     GetMainFrame()->GetRenderWidgetHost()->SetPageFocus(true);
   }
 }
 
 void WebContentsImpl::SetFocusedFrame(FrameTreeNode* node,
                                       SiteInstance* source) {
   SetAsFocusedWebContentsIfNecessary();
   frame_tree_.SetFocusedFrame(node, source);
 }
 
+void WebContentsImpl::DidChangeOpener(FrameTreeNode* node,
+                                      FrameTreeNode* opener) {
+  if (!node->IsMainFrame())
+    return;
+
+  DCHECK_EQ(GetOpener(), opener->current_frame_host());
+  for (auto& observer : observers_)
+    observer.DidChangeOpener();
+}
+
 void WebContentsImpl::OnFocusedElementChangedInFrame(
     RenderFrameHostImpl* frame,
     const gfx::Rect& bounds_in_root_view) {
   RenderWidgetHostViewBase* root_view =
       static_cast<RenderWidgetHostViewBase*>(GetRenderWidgetHostView());
   if (!root_view || !frame->GetView())
     return;
 
   // Converting to screen coordinates.
   gfx::Point origin = bounds_in_root_view.origin();
@@ skipping 673 matching lines @@
     new_root->SetOriginalOpener(opener->frame_tree()->root());
 
     if (!opener_suppressed) {
       new_root->SetOpener(opener);
       created_with_opener_ = true;
     }
   }
 }
 
 }  // namespace content