Add the IDL for chrome.instanceID API.

chrome/common/extensions/api/_api_features.json

Index: chrome/common/extensions/api/_api_features.json
diff --git a/chrome/common/extensions/api/_api_features.json b/chrome/common/extensions/api/_api_features.json
index c8b4fc64eb0cc3394cb9699880c42e076e358ccf..6bd884bbc628582ae3e527dc10caea4ba15e7b8e 100644
--- a/chrome/common/extensions/api/_api_features.json
+++ b/chrome/common/extensions/api/_api_features.json
@@ -495,6 +495,10 @@
     "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"]