Avoid exposing cc::Layer tree to CompositorProxy

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

Index: third_party/WebKit/Source/platform/graphics/CompositorMutableProperties.h
diff --git a/third_party/WebKit/Source/platform/graphics/CompositorMutableProperties.h b/third_party/WebKit/Source/platform/graphics/CompositorMutableProperties.h
index 8aecbfff4cd2e741e0bb36f53513ce457ca64aa6..3c4db3f3f3b132c7d20fc3d95ac64d14d7883ca6 100644
--- a/third_party/WebKit/Source/platform/graphics/CompositorMutableProperties.h
+++ b/third_party/WebKit/Source/platform/graphics/CompositorMutableProperties.h
@@ -5,12 +5,31 @@
 #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;
 
+// Holds the values of the available mutable properties for a |CompositorProxy|.
+// Provided to a proxy at the start of a mutate call with the current input
+// values taken from the layer tree.
+//
+// This data structure is not responsible for tracking property mutations; for
+// that see |CompositorMutation|.
+class CompositorMutableProperties {
+ public:
+  uint64_t element_id;
+  double opacity;
+  float scroll_left;
+  float scroll_top;
+  SkMatrix44 transform;
+};
+typedef std::unordered_map<uint64_t, CompositorMutableProperties>
+    ProxyCompositorMutablePropertiesMap;
+
 }  // namespace blink
 
 #endif  // CompositorMutableProperties_h