Log to UMA when x-origin frames (1) are created and (2) become visible

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 4242 matching lines @@
 
 void FrameView::notifyRenderThrottlingObservers()
 {
     TRACE_EVENT0("blink", "FrameView::notifyRenderThrottlingObservers");
     DCHECK(!isInPerformLayout());
     DCHECK(!m_frame->document() || !m_frame->document()->inStyleRecalc());
     bool wasThrottled = canThrottleRendering();
 
     updateThrottlingStatus();
 
+    if (frame().document())

[dcheng, 2016/08/29 20:37:19]

Do we need to do this check because Widget isn't synchronously detached (see
UpdateSuspendScope)?

[dgrogan, 2016/08/31 19:52:41]

Not sure... I was cargo-culting from the first clause of line 4258's DCHECK that
implies frame().document() can be null.

[dcheng, 2016/09/01 21:24:26]

Looking at the code, I'm actually not sure how we'd end up in this situation. I
believe this task is only ever posted when we are attached (e.g. frame() is not
null) and the task itself is cancelled when the FrameView is disposed.

Can you try just asserting that frame().document() is non-null and see what
happens?

+        frame().document()->onVisibilityMaybeChanged(!m_hiddenForThrottling);
+
     bool becameThrottled = !wasThrottled && canThrottleRendering();
     bool becameUnthrottled = wasThrottled && !canThrottleRendering();
     ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator();
     if (becameThrottled) {
         // If this FrameView became throttled, we must make sure all of its
         // children become throttled at the same time. Otherwise we might
         // attempt to paint one of the children with an out-of-date layout
         // before |notifyRenderThrottlingObservers| has made it throttled.
         forAllNonThrottledFrameViews([](FrameView& frameView) {
             frameView.m_subtreeThrottled = true;
@@ skipping 35 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const
 {
     if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
         return false;
     return m_subtreeThrottled || (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
 } // namespace blink