Only deliver extension messages to contexts that care.

chrome/renderer/resources/renderer_extension_bindings.js

Index: chrome/renderer/resources/renderer_extension_bindings.js
diff --git a/chrome/renderer/resources/renderer_extension_bindings.js b/chrome/renderer/resources/renderer_extension_bindings.js
index 0c7ef88e963fde91352b0b09ba86c1e70c3a6e03..52327c3578ec2b2b4d66cc0f0784ec88310d58c5 100644
--- a/chrome/renderer/resources/renderer_extension_bindings.js
+++ b/chrome/renderer/resources/renderer_extension_bindings.js
@@ -46,6 +46,13 @@ var chrome = chrome || {};
 
   chromeHidden.Port = {};
 
+  // Returns true if the specified port id is in this context. This is used by
+  // the C++ to avoid creating the javascript message for all the contexts that
+  // don't care about a particular message.
+  chromeHidden.Port.hasPort = function(portId) {
+    return portId in ports;
+  };
+
   // Hidden port creation function.  We don't want to expose an API that lets
   // people add arbitrary port IDs to the port list.
   chromeHidden.Port.createPort = function(portId, opt_name) {