Clip related property and border radius are now recorded for UMA

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index d5748a61a533ed93610300bda459836bfcd96972..60ed111fef4b4ecd4da4ae61e1126d0338941cd8 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1736,6 +1736,8 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
         CompositorMutableProperty::kScrollLeft)))
     return true;
 
+  bool needsCompositedScrolling = true;
+
   // TODO(flackr): Allow integer transforms as long as all of the ancestor
   // transforms are also integer.
   bool backgroundSupportsLCDText =
@@ -1762,17 +1764,28 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
       addStyleRelatedMainThreadScrollingReasons(
           MainThreadScrollingReason::kBackgroundNotOpaqueInRect);
     }
-    return false;
+    needsCompositedScrolling = false;
   }
 
   // TODO(schenney) Tests fail if we do not also exclude
   // layer->layoutObject()->style()->hasBorderDecoration() (missing background
   // behind dashed borders). Resolve this case, or not, and update this check
   // with the results.
-  return !(layer->size().isEmpty() || layer->hasDescendantWithClipPath() ||
-           layer->hasAncestorWithClipPath() ||
-           layer->layoutObject()->style()->hasBorderRadius() ||
-           layer->layoutObject()->hasClip());
+  if (layer->size().isEmpty())

[bokan, 2017/01/05 14:05:48]

It's inconsistent that if the size is empty, we'll add some but not all the main
thread scrolling reasons. I'd move this up to before you declare
needsCompositedScrolling.

[yigu, 2017/01/05 14:46:51]

Done.

+    return false;
+  if (layer->layoutObject()->style()->hasBorderRadius()) {
+    addStyleRelatedMainThreadScrollingReasons(
+        MainThreadScrollingReason::kHasBorderRadius);
+    needsCompositedScrolling = false;
+  }
+  if (layer->layoutObject()->hasClip() ||
+      layer->layoutObject()->hasClipPath() ||
+      layer->hasDescendantWithClipPath() || layer->hasAncestorWithClipPath()) {
+    addStyleRelatedMainThreadScrollingReasons(
+        MainThreadScrollingReason::kHasClipRelatedProperty);
+    needsCompositedScrolling = false;
+  }
+  return needsCompositedScrolling;
 }
 
 void PaintLayerScrollableArea::addStyleRelatedMainThreadScrollingReasons(