Initialize content scripts the same way extension scripts are.

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 b9811c1cb70be74a663a3f5c4993974006550786..2550b2462557e568350d1d6418ab81c3566779a9 100644
--- a/chrome/renderer/resources/renderer_extension_bindings.js
+++ b/chrome/renderer/resources/renderer_extension_bindings.js
@@ -173,9 +173,8 @@ var chrome = chrome || {};
 
   // This function is called on context initialization for both content scripts
   // and extension contexts.
-  chrome.initExtension = function(extensionId, warnOnPrivilegedApiAccess,
-                                  inIncognitoContext) {
-    delete chrome.initExtension;
+  chromeHidden.onLoad.addListener(function(extensionId, isExtensionProcess,
+                                           inIncognitoContext) {
     chromeHidden.extensionId = extensionId;
 
     chrome.extension = chrome.extension || {};
@@ -259,10 +258,9 @@ var chrome = chrome || {};
       return GetL10nMessage(message_name, placeholders, extensionId);
     };
 
-    if (warnOnPrivilegedApiAccess) {
+    if (!isExtensionProcess)
       setupApiStubs();
-    }
-  };
+  });
 
   var notSupportedSuffix = " is not supported in content scripts. " +
       "See the content scripts documentation for more details.";