compositor-worker: Add mutable attributes to CompositorProxy.

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

Index: LayoutTests/fast/compositorworker/resources/proxy-mutate.js
diff --git a/LayoutTests/fast/compositorworker/resources/proxy-mutate.js b/LayoutTests/fast/compositorworker/resources/proxy-mutate.js
new file mode 100644
index 0000000000000000000000000000000000000000..3089a48ec3601a3a2b37532e9212bd3ef17806a2
--- /dev/null
+++ b/LayoutTests/fast/compositorworker/resources/proxy-mutate.js
@@ -0,0 +1,14 @@
+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);
+    }
+}