Land layer squashing behind a flag

Source/core/rendering/CompositingReasons.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 CompositingReasons_h
 #define CompositingReasons_h
 
 #include "wtf/MathExtras.h"
 #include <stdint.h>
 
@@ skipping 44 matching lines @@
 const uint64_t CompositingReasonLayerForMask                           = UINT64_C(1) << 31;
 
 // FIXME: the following compositing reasons need to be re-organized to fit with categories
 // used in all the other reasons above.
 const uint64_t CompositingReasonOverflowScrollingParent                = UINT64_C(1) << 32;
 const uint64_t CompositingReasonOutOfFlowClipping                      = UINT64_C(1) << 33;
 
 const uint64_t CompositingReasonLayerForVideoOverlay                   = UINT64_C(1) << 34;
 const uint64_t CompositingReasonIsolateCompositedDescendants           = UINT64_C(1) << 35;
 
+// Note: if you add more reasons here, you will need to update WebCompositingReasons as well.
+
+// Various combinations of compositing reasons are defined here also, for more intutive and faster bitwise logic.
 const uint64_t CompositingReasonComboAllDirectReasons =
     CompositingReason3DTransform
     | CompositingReasonVideo
     | CompositingReasonCanvas
     | CompositingReasonPlugin
     | CompositingReasonIFrame
     | CompositingReasonBackfaceVisibilityHidden
     | CompositingReasonAnimation
     | CompositingReasonFilters
     | CompositingReasonPositionFixed
     | CompositingReasonPositionSticky
     | CompositingReasonOverflowScrollingTouch
     | CompositingReasonOverflowScrollingParent
     | CompositingReasonOutOfFlowClipping;
 
 const uint64_t CompositingReasonComboReasonsThatRequireOwnBacking =
     CompositingReasonComboAllDirectReasons
     | CompositingReasonOverlap
     | CompositingReasonAssumedOverlap
     | CompositingReasonNegativeZIndexChildren
     | CompositingReasonTransformWithCompositedDescendants
     | CompositingReasonOpacityWithCompositedDescendants
     | CompositingReasonMaskWithCompositedDescendants
     | CompositingReasonFilterWithCompositedDescendants
     | CompositingReasonBlendingWithCompositedDescendants
     | CompositingReasonIsolateCompositedDescendants
     | CompositingReasonPreserve3D; // preserve-3d has to create backing store to ensure that 3d-transformed elements intersect.
 
-// Note: if you add more reasons here, you will need to update WebCompositingReasons as well.
+const uint64_t CompositingReasonComboAllOverlapReasons =
+    CompositingReasonOverlap
+    | CompositingReasonAssumedOverlap;
+
 typedef uint64_t CompositingReasons;
 
 
 } // namespace WebCore
 
 #endif // CompositingReasons_h