Add more specific metrics for main thread scrolling reasons

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

Index: third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp
diff --git a/third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp b/third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp
index a2194782061ed57c8b036b8c1d5418d3dfd5a2c3..df56ca0b2bba2114ae7708c177e0e01e287094eb 100644
--- a/third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp
+++ b/third_party/WebKit/Source/web/tests/ScrollingCoordinatorTest.cpp
@@ -849,4 +849,38 @@ TEST_F(ScrollingCoordinatorTest, CustomScrollbarShouldTriggerMainThreadScroll) {
       MainThreadScrollingReason::kCustomScrollbarScrolling);
 }
 
+TEST_F(ScrollingCoordinatorTest, MainThreadScrollingReasonDueToLayoutObject) {
+  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.
+  // 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