Add the IDL for chrome.instanceID API.

chrome/common/extensions/api/_api_features.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.
 
 // This features file defines extension APIs implemented under src/chrome.
 // See extensions/common/features/* to understand this file, in particular
 // feature.h, simple_feature.h, and base_feature_provider.h.
 //
 // Note that specifying "web_page", "blessed_web_page", or "all" as a context
 // type will require manually updating chrome/renderer/resources/dispatcher.cc.
@@ skipping 477 matching lines @@
     "contexts": ["blessed_extension"]
   },
   "input.ime": {
     "dependencies": ["permission:input"],
     "contexts": ["blessed_extension"]
   },
   "inputMethodPrivate": {
     "dependencies": ["permission:inputMethodPrivate"],
     "contexts": ["blessed_extension"]
   },
+  "instanceID": {
+    "dependencies": ["permission:gcm"],

[fgorski, 2015/04/22 17:34:59]

Is this choice conscious?
When we start using Instance ID beyond GCM and someone does not use GCM, will
you still require GCM permission?

Given that to take advantage of the API, developers will make changes in the
code, there is a chance to add permission at that time.

[jianli, 2015/04/22 20:18:20]

For now I think it would be better to put it under GCM permission since we need
to get the device ID in order to talk with the server. We can update the
permission upon more discussion.

[not at google - send to devlin, 2015/04/22 20:47:42]

If you add other APIs that depend on instanceID you can add multiple rules which
grant access to instanceID based on those, too.

+    "contexts": ["blessed_extension"]
+  },
   "launcherPage": {
     "dependencies": ["manifest:launcher_page"],
     "contexts": ["blessed_extension"]
   },
   "launcherSearchProvider": {
     "dependencies": ["permission:launcherSearchProvider"],
     "contexts": ["blessed_extension"]
   },
   "location": {
     "dependencies": ["permission:location"],
@@ skipping 219 matching lines @@
     "internal": true,
     "channel": "stable",
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }