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

Source/core/layout/compositing/CompositingReasonFinder.cpp

Index: Source/core/layout/compositing/CompositingReasonFinder.cpp
diff --git a/Source/core/layout/compositing/CompositingReasonFinder.cpp b/Source/core/layout/compositing/CompositingReasonFinder.cpp
index 98fc176b091f3053aa746724389aaa50477fae83..de049dc92ac31ed546a4810faae2b8affeb3e86b 100644
--- a/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -110,6 +110,9 @@ CompositingReasons CompositingReasonFinder::potentialCompositingReasonsFromStyle
     if (style.hasScrollBlocksOn() && !renderer->isDocumentElement())
         reasons |= CompositingReasonScrollBlocksOn;
 
+    if (style.hasCompositorProxy())
+        reasons |= CompositingReasonCompositorProxy;
+
     // If the implementation of createsGroup changes, we need to be aware of that in this part of code.
     ASSERT((renderer->isTransparent() || renderer->hasMask() || renderer->hasFilter() || style.hasBlendMode()) == renderer->createsGroup());