Record non-stacking-context as main thread scrolling reasons

third_party/WebKit/Source/core/paint/PaintLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 570 matching lines @@
 
   // Returns true if the element or any ancestor has non 1 opacity.
   bool CompositesWithOpacity() const;
 
   bool PaintsWithTransform(GlobalPaintFlags) const;
 
   bool SupportsSubsequenceCaching() const;
 
   // Returns true if background phase is painted opaque in the given rect.
   // The query rect is given in local coordinates.
-  bool BackgroundIsKnownToBeOpaqueInRect(const LayoutRect&) const;
+  // Under certain circumstances we need to know the detailed reasons that
+  // why it returns false.

[bokan, 2017/04/26 15:52:15]

Under which circumstance? It's in order to record UMA stats about why we didn't
composite.

[yigu, 2017/04/27 15:28:27]

Done.

[flackr, 2017/04/27 18:42:35]

New patchset?

+  bool BackgroundIsKnownToBeOpaqueInRect(const LayoutRect&,
+                                         uint32_t* reasons = nullptr) const;
 
   bool ContainsDirtyOverlayScrollbars() const {
     return contains_dirty_overlay_scrollbars_;
   }
   void SetContainsDirtyOverlayScrollbars(bool dirty_scrollbars) {
     contains_dirty_overlay_scrollbars_ = dirty_scrollbars;
   }
 
   CompositorFilterOperations CreateCompositorFilterOperationsForFilter(
       const ComputedStyle&);
@@ skipping 675 matching lines @@
 
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLayerTree(const blink::PaintLayer*);
 CORE_EXPORT void showLayerTree(const blink::LayoutObject*);
 #endif
 
 #endif  // Layer_h