Don't expose a LayerImpl reference to CompositorProxy.

third_party/WebKit/Source/core/dom/CompositorProxy.h

Index: third_party/WebKit/Source/core/dom/CompositorProxy.h
diff --git a/third_party/WebKit/Source/core/dom/CompositorProxy.h b/third_party/WebKit/Source/core/dom/CompositorProxy.h
index 37efc8446b9c7ccaee8fc3a31b967010ae1fd374..dd2ee5b3726a550e8492e3653b23a4f66e647961 100644
--- a/third_party/WebKit/Source/core/dom/CompositorProxy.h
+++ b/third_party/WebKit/Source/core/dom/CompositorProxy.h
@@ -37,6 +37,7 @@ class CORE_EXPORT CompositorProxy final
 
   DEFINE_INLINE_TRACE() {}
 
+  uint64_t proxyId() const { return m_proxyId; }
   uint64_t elementId() const { return m_elementId; }
   uint32_t compositorMutableProperties() const {
     return m_compositorMutableProperties;
@@ -72,6 +73,7 @@ class CORE_EXPORT CompositorProxy final
   void disconnectInternal();
 
   const uint64_t m_elementId = 0;
+  const uint64_t m_proxyId = 0;
   const uint32_t m_compositorMutableProperties = 0;
 
   bool m_connected = true;