Record non-stacking-context as main thread scrolling reasons

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 ce48c55fb9b17b064537b3b24b6160eee2b6f83b..4d6f5d2aa89aa8fa2175a887b450267f003bf41e 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1848,10 +1848,15 @@ bool PaintLayerScrollableArea::ComputeNeedsCompositedScrolling(
           MainThreadScrollingReason::kHasTransformAndLCDText;
     }
     if (!layer->BackgroundIsKnownToBeOpaqueInRect(
-            ToLayoutBox(layer->GetLayoutObject()).PaddingBoxRect())) {
+            ToLayoutBox(layer->GetLayoutObject()).PaddingBoxRect(),
+            &non_composited_main_thread_scrolling_reasons_)) {
       non_composited_main_thread_scrolling_reasons_ |=
           MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText;
     }
+    if (!layer->GetLayoutObject().Style()->IsStackingContext()) {
+      non_composited_main_thread_scrolling_reasons_ |=
+          MainThreadScrollingReason::kIsNotStackingContextAndLCDText;
+    }
 
     needs_composited_scrolling = false;
   }