compositor-worker: Force elements to grow a layer when a CompositorProxy is created.

Source/platform/graphics/CompositingReasons.cpp

Index: Source/platform/graphics/CompositingReasons.cpp
diff --git a/Source/platform/graphics/CompositingReasons.cpp b/Source/platform/graphics/CompositingReasons.cpp
index 742d89041ba90f0b684638636574d0b287b52fde..f64fa8d54b0713d6ba9e7ca968c2da087348047b 100644
--- a/Source/platform/graphics/CompositingReasons.cpp
+++ b/Source/platform/graphics/CompositingReasons.cpp
@@ -58,6 +58,9 @@ const CompositingReasonStringMap kCompositingReasonStringMap[] = {
     { CompositingReasonScrollBlocksOn,
         "scrollBlocksOn",
         "Has a CSS scroll-blocks-on value other than none" },
+    { CompositingReasonCompositorProxy,
+        "compositorProxy",
+        "Has a CompositorProxy object" },
     { CompositingReasonAssumedOverlap,
         "assumedOverlap",
         "Might overlap other composited content" },