Re-enable ScrollbarAnimationController::DidRequestShowFromMainThread.

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 1650 matching lines @@
       if (root_layer_scrolling_enabled) {
         layout_view.Layer()->SetNeedsCompositingInputsUpdate();
         if (RuntimeEnabledFeatures::slimmingPaintInvalidationEnabled())
           SetNeedsPaintPropertyUpdate();
       } else {
         layout_view.Compositor()->FrameViewDidChangeSize();
       }
     }
   }
 
+// TODO(skobes): For reasons not fully understood, showing scrollbars from
+// main on viewport size changes regresses memory and perf tests on Android

[aelias_OOO_until_Jul13, 2017/04/25 21:05:08]

Hmm, since you traced it to viewport sized changes, it's probably due to top
controls hiding in some way.  +cc bokan@, any idea what the problem might be?

+// (crbug.com/706927, crbug.com/712453).
+#if !OS(ANDROID)

[aelias_OOO_until_Jul13, 2017/04/25 21:05:08]

I really don't like OS(ANDROID), among other things because it winds up forcing
Android-specific unit tests or unit tests that only go red on Android.  If we're
going to go with platform-specific behavior, I prefer it to be behind a
WebSetting that's only true on Android.  A natural one for this purpose would be
GetPage()->GetSettings().GetMainFrameResizesAreOrientationChanges()

   ShowOverlayScrollbars();
+#endif
 
   if (root_layer_scrolling_enabled) {
     // The background must be repainted when the FrameView is resized, even if
     // the initial containing block does not change (so we can't rely on layout
     // to issue the invalidation).  This is because the background fills the
     // main GraphicsLayer, which takes the size of the layout viewport.
     // TODO(skobes): Paint non-fixed backgrounds into the scrolling contents
     // layer and avoid this invalidation (http://crbug.com/568847).
     LayoutViewItem lvi = GetLayoutViewItem();
     if (!lvi.IsNull())
@@ skipping 3694 matching lines @@
 void FrameView::SetAnimationHost(
     std::unique_ptr<CompositorAnimationHost> host) {
   animation_host_ = std::move(host);
 }
 
 LayoutUnit FrameView::CaretWidth() const {
   return LayoutUnit(GetChromeClient()->WindowToViewportScalar(1));
 }
 
 }  // namespace blink