Add chrome.inputMethodPrivate.onImeMenuActivationChanged API.

chrome/common/extensions/api/input_method_private.json

 // Copyright (c) 2012 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": "inputMethodPrivate",
     "description": "none",
     "functions": [
       {
@@ skipping 207 matching lines @@
             }
           }, {
             "name": "removed",
             "type": "array",
             "description": "List of removed words.",
             "items": {
               "type": "string"
             }
           }
         ]
+      }, {
+        "name": "onImeMenuActivationChanged",
+        "type": "function",
+        "description": "Fired when the system IME Menu is activated",
+        "parameters": [
+          {
+            "name": "activation",
+            "type": "boolean",
+            "description": "Whether the IME menu is activated or deactivated"
+          }
+        ]
       }
     ]
   }
 ]