Record non-stacking-context as main thread scrolling reasons

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 1f9148f70c35bc50321b277d0a6b01410f2b6fb7..311675ddfb47cc0f115fd79efa212c5a7128e28c 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -2791,8 +2791,8 @@ bool PaintLayer::CompositesWithOpacity() const {
   return OpacityAncestor() || GetLayoutObject().Style()->HasOpacity();
 }
 
-bool PaintLayer::BackgroundIsKnownToBeOpaqueInRect(
-    const LayoutRect& local_rect) const {
+bool PaintLayer::BackgroundIsKnownToBeOpaqueInRect(const LayoutRect& local_rect,
+                                                   uint32_t* reasons) const {
   if (PaintsWithTransparency(kGlobalPaintNormalPhase))
     return false;
 
@@ -2830,8 +2830,13 @@ bool PaintLayer::BackgroundIsKnownToBeOpaqueInRect(
 
   // We can't consult child layers if we clip, since they might cover
   // parts of the rect that are clipped out.
-  if (GetLayoutObject().HasClipRelatedProperty())
+  if (GetLayoutObject().HasClipRelatedProperty()) {
+    if (reasons && GetLayoutObject().HasOverflowClip()) {
+      *reasons |= MainThreadScrollingReason::
+          kHasOverflowClipAndLayoutObjectBackgroundNotOpaqueAndLCDText;
+    }
     return false;
+  }
 
   // TODO(schenney): This could be improved by unioning the opaque regions of
   // all the children.  That would require a refactoring because currently