Land layer squashing behind a flag

Source/core/rendering/CompositingState.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CompositingState_h
 #define CompositingState_h
 
 namespace WebCore {
 
 enum CompositingState {
     // The layer paints into its enclosing composited ancestor.
     NotComposited = 0,
 
     // The layer is composited, but its contents still paint into enclosing composited ancestor.
     // In this state, repaint invalidations must be sent to the enclosing composited ancestor.
     // Typically this happens when a layer's properties need to be represented in the compositor
     // output data structures, but it doesn't actually have any other reasons to be composited.
     HasOwnBackingButPaintsIntoAncestor = 1,
 
     PaintsIntoOwnBacking = 2,
+
+    // In this state, the RenderLayer subtree paints into a backing that is shared by
+    // several RenderLayer subtrees.
+    PaintsIntoGroupedBacking = 3
 };
 
 } // namespace WebCore
 
 #endif // CompositingState_h