Plumb CompositingReason from Blink to compositor.

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index 735a870fcbe8d13295ec51ecfac61c3475df2e72..ecb77f512a1ac73f42c70f06a0f798a1035f712d 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -117,7 +117,16 @@ enum {
 
     // The root layer is a special case that may be forced to be a layer, but also it needs to be
     // a layer if anything else in the subtree is composited.
-    CompositingReasonRoot                                   = 1 << 26
+    CompositingReasonRoot                                   = 1 << 26,
+
+    // RenderLayerBacking internal hierarchy reasons
+    CompositingReasonLayerForClip                           = 1 << 27,
+    CompositingReasonLayerForDescendantClip                 = 1 << 28,
+    CompositingReasonLayerForScrollbar                      = 1 << 29,
+    CompositingReasonLayerForScrollingContainer             = 1 << 30,
+    CompositingReasonLayerForForeground                     = 1 << 31
+
+    // Note: if you add more reasons here, you will need to update the WebCompositingReasons enum as well.
 };
 typedef unsigned CompositingReasons;