Add more specific metrics for main thread scrolling reasons

third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 831 matching lines @@
   container->removeAttribute("class");
   forceFullCompositingUpdate();
   scrollbarGraphicsLayer = compositedLayerMapping->layerForVerticalScrollbar();
   ASSERT_FALSE(
       scrollbarGraphicsLayer->platformLayer()->shouldScrollOnMainThread());
   ASSERT_FALSE(
       scrollbarGraphicsLayer->platformLayer()->mainThreadScrollingReasons() &
       MainThreadScrollingReason::kCustomScrollbarScrolling);
 }
 
+TEST_F(ScrollingCoordinatorTest, MainThreadScrollingReasonFromStyle) {
+  registerMockedHttpURLLoad("two_transparent_scrollable_area.html");
+  navigateTo(m_baseURL + "two_transparent_scrollable_area.html");
+  forceFullCompositingUpdate();
+
+  Page* page = frame()->page();
+  ASSERT_TRUE(page->scrollingCoordinator());
+  ASSERT_TRUE(page->scrollingCoordinator()->mainThreadScrollingReasons() &
+              MainThreadScrollingReason::kHasOpacity);
+
+  // Remove opacity from one of the layouts.

[bokan, 2016/12/14 13:28:59]

layouts->scrollers

[yigu, 2016/12/14 21:17:33]

Done.

+  // Still need to scroll on main thread.
+  Document* document = frame()->document();
+  Element* container = document->getElementById("scroller1");
+  DCHECK(container);
+
+  container->removeAttribute("class");
+  forceFullCompositingUpdate();
+
+  ASSERT_TRUE(page->scrollingCoordinator()->mainThreadScrollingReasons() &
+              MainThreadScrollingReason::kHasOpacity);
+
+  // Remove opacity from the other layout would lead to
+  // scroll on impl.
+  container = document->getElementById("scroller2");
+  DCHECK(container);
+
+  container->removeAttribute("class");
+  forceFullCompositingUpdate();
+
+  ASSERT_FALSE(page->scrollingCoordinator()->mainThreadScrollingReasons() &
+               MainThreadScrollingReason::kHasOpacity);
+}
+
 }  // namespace blink