Plumb CompositingReason from Blink to compositor.

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 99 matching lines @@
     CompositingReasonReflectionWithCompositedDescendants    = 1 << 19,
     CompositingReasonFilterWithCompositedDescendants        = 1 << 20,
     CompositingReasonBlendingWithCompositedDescendants      = 1 << 21,
     CompositingReasonClipsCompositingDescendants            = 1 << 22,
     CompositingReasonPerspective                            = 1 << 23,
     CompositingReasonPreserve3D                             = 1 << 24,
     CompositingReasonReflectionOfCompositedParent           = 1 << 25,
 
     // 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;
 
 class ClipRect {
 public:
     ClipRect()
     : m_hasRadius(false)
     { }
     
     ClipRect(const LayoutRect& rect)
@@ skipping 1210 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const WebCore::RenderLayer*);
 void showLayerTree(const WebCore::RenderObject*);
 #endif
 
 #endif // RenderLayer_h