Don't expose a LayerImpl reference to CompositorProxy.

third_party/WebKit/Source/platform/graphics/CompositorMutableProperties.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CompositorMutableProperties_h
 #define CompositorMutableProperties_h
 
+#include <unordered_map>
 #include "cc/trees/mutable_properties.h"
+#include "third_party/skia/include/core/SkMatrix44.h"
 
 namespace blink {
 
 using CompositorMutableProperty = cc::MutableProperty;
 
+class CompositorMutableProperties {
+ public:
+  uint64_t elementId;
+  double opacity;
+  float scrollLeft;
+  float scrollTop;
+  SkMatrix44 transform;
+};
+typedef std::unordered_map<uint64_t, CompositorMutableProperties>
+    ProxyCompositorMutablePropertiesMap;
+
 }  // namespace blink
 
 #endif  // CompositorMutableProperties_h