compositor-worker: Add mutable attributes to CompositorProxy.

LayoutTests/fast/compositorworker/resources/proxy-mutate.js

+onmessage = function(msg) {
+    var proxy = msg.data[0];
+    var attrib = msg.data[1];
+    try {
+        if (proxy.supports(attrib)) {
+            proxy[attrib] = 0;
+            postMessage('success');
+        } else {
+            postMessage('error: Received non-supported attribute "' + attrib + '"');
+        }
+    } catch (e) {
+        postMessage('error: ' + e);
+    }
+}