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 21a545ed56bb0face64e0100b3480bf15338bf16..059156c9d2a8d1602af460fc13dc1d3fa56205ea 100644
--- a/Source/core/rendering/CompositingReasons.h
+++ b/Source/core/rendering/CompositingReasons.h
@@ -61,6 +61,9 @@ const uint64_t CompositingReasonOutOfFlowClipping                      = UINT64_
 
 const uint64_t CompositingReasonLayerForVideoOverlay                   = UINT64_C(1) << 34;
 
+// 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
@@ -88,7 +91,10 @@ const uint64_t CompositingReasonComboReasonsThatRequireOwnBacking =
     | CompositingReasonBlendingWithCompositedDescendants
     | 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;