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 39eac7dd2eece7af896d726f85017b5fb2855743..883b7ea612c52f793c98ab23d1e713a6fb3c5de0 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1823,10 +1823,15 @@ 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;
   }