Add extension event histogram values for messaging, webRequest, and webview.

extensions/renderer/resources/extension_custom_bindings.js

Index: extensions/renderer/resources/extension_custom_bindings.js
diff --git a/extensions/renderer/resources/extension_custom_bindings.js b/extensions/renderer/resources/extension_custom_bindings.js
index d114f527f56e92418cb4c8b54a4623b7d3a7b145..155b7625405f6ca0dcf38b77ab04eafdf01323c5 100644
--- a/extensions/renderer/resources/extension_custom_bindings.js
+++ b/extensions/renderer/resources/extension_custom_bindings.js
@@ -9,8 +9,6 @@ var binding = require('binding').Binding.create('extension');
 var messaging = require('messaging');
 var runtimeNatives = requireNative('runtime');
 var GetExtensionViews = runtimeNatives.GetExtensionViews;
-var OpenChannelToExtension = runtimeNatives.OpenChannelToExtension;
-var OpenChannelToNativeApp = runtimeNatives.OpenChannelToNativeApp;
 var chrome = requireNative('chrome').GetChrome();
 
 var inIncognitoContext = requireNative('process').InIncognitoContext();