compositor-worker: Add mutable attributes to CompositorProxy.

Source/core/dom/CompositorProxy.idl

Index: Source/core/dom/CompositorProxy.idl
diff --git a/Source/core/dom/CompositorProxy.idl b/Source/core/dom/CompositorProxy.idl
index 021ee6586aec6968b7022f094a7f7f56e1854b59..924362c63edff9625c32fb55549d4a77a5d97864 100644
--- a/Source/core/dom/CompositorProxy.idl
+++ b/Source/core/dom/CompositorProxy.idl
@@ -10,5 +10,10 @@
     RaisesException=Constructor,
     RuntimeEnabled=CompositorWorker,
 ] interface CompositorProxy {
+    [RaisesException] attribute double opacity;
+    [RaisesException] attribute double scrollLeft;
+    [RaisesException] attribute double scrollTop;
+    [RaisesException] attribute DOMMatrix transform;
+
     boolean supports(DOMString attribute);
 };