compositor-worker: Introduce 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
new file mode 100644
index 0000000000000000000000000000000000000000..021ee6586aec6968b7022f094a7f7f56e1854b59
--- /dev/null
+++ b/Source/core/dom/CompositorProxy.idl
@@ -0,0 +1,14 @@
+// Copyright 2015 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.
+
+[
+    Constructor(Element element, DOMString[] attributeArray),
+    ConstructorCallWith=ExecutionContext,
+    Exposed=Window,
+    GarbageCollected,
+    RaisesException=Constructor,
+    RuntimeEnabled=CompositorWorker,
+] interface CompositorProxy {
+    boolean supports(DOMString attribute);
+};