Land layer squashing behind a flag

Source/core/rendering/CompositingReasons.h

Index: Source/core/rendering/CompositingReasons.h
diff --git a/Source/core/rendering/CompositingReasons.h b/Source/core/rendering/CompositingReasons.h
index b07999070b27507415902b218d9fd505954707b0..8b023a724f8f990c8e1c5eaee206b327585712e3 100644
--- a/Source/core/rendering/CompositingReasons.h
+++ b/Source/core/rendering/CompositingReasons.h
@@ -62,6 +62,9 @@ const uint64_t CompositingReasonOutOfFlowClipping                      = UINT64_
 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
@@ -90,7 +93,10 @@ const uint64_t CompositingReasonComboReasonsThatRequireOwnBacking =
     | 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;