Added JsonMessage to the control channel.

remoting/webapp/client_plugin_async.js

Index: remoting/webapp/client_plugin_async.js
diff --git a/remoting/webapp/client_plugin_async.js b/remoting/webapp/client_plugin_async.js
index f457d159fe466b838744443b4fe2f846891b8271..ac82c9f914177d86e01e400e508e167c150b0621 100644
--- a/remoting/webapp/client_plugin_async.js
+++ b/remoting/webapp/client_plugin_async.js
@@ -313,6 +313,10 @@ remoting.ClientPluginAsync.prototype.handleMessage_ = function(messageStr) {
       return;
     }
     this.onPairingComplete_(clientId, sharedSecret);
+  } else if (message.method == 'extensionMessage') {
+    // No messages currently supported.
+    console.log('Unexpected message received: ' +
+                message.data.type + ': ' + message.data.data);
   }
 };
 
@@ -607,6 +611,23 @@ remoting.ClientPluginAsync.prototype.requestPairing =
 };
 
 /**
+ * Send an extension message to the host.
+ *
+ * @param {string} type The message type.
+ * @param {Object} message The message payload.
+ */
+remoting.ClientPluginAsync.prototype.sendClientMessage =
+    function(type, message) {
+  if (!this.hasFeature(remoting.ClientPlugin.Feature.EXTENSION_MESSAGE)) {
+    return;
+  }
+  this.plugin.postMessage(JSON.stringify(
+    { method: 'extensionMessage',
+      data: { type: type, data: JSON.stringify(message) } }));
+
+};
+
+/**
  * If we haven't yet received a "hello" message from the plugin, change its
  * size so that the user can confirm it if click-to-play is enabled, or can
  * see the "this plugin is disabled" message if it is actually disabled.