<webview>: make context menus cancellable using JS API

chrome/common/extensions/api/chrome_web_view_internal.json

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace": "chromeWebViewInternal",
     "description": "none",
     "compiler_options": {
       "implemented_in": "chrome/browser/extensions/api/web_view/chrome_web_view_internal_api.h"
@@ skipping 270 matching lines @@
           }
         ]
       }
     ],
     "events": [
       {
         "name": "onClicked",
         "type": "function",
         "nodoc": true,
         "$ref": "contextMenusInternal.onClicked"
+      },
+      {
+        "name": "onContextMenu",
+        "type": "function",
+        "nodoc": true,
+        "parameters": [
+          {
+            "name": "event",
+            "type": "object",
+            "properties": {
+              "preventDefault": {
+                "type": "function",
+                "parameters": [
+                ]
+              }
+            }
+          }
+        ]
       }
     ]
   }
 ]