Record non-stacking-context as main thread scrolling reasons

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.h b/third_party/WebKit/Source/core/paint/PaintLayer.h
index 83691b8b2a5b1467e59f4bee882dd28c06d63e57..b8091b9f6aee6fed8669d3ecf325bb39932fdc15 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -588,7 +588,10 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
 
   // 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_;