Identity API: Add chrome.identity.onSignInChanged routing and IDL

chrome/common/extensions/api/_permission_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.
 
 // See c/c/e/features/* to understand this file, in particular feature.h,
 // simple_feature.h, and base_feature_provider.h.
 
 {
   "activeTab": {
     "channel": "stable",
@@ skipping 330 matching lines @@
     ]
   },
   "history": {
     "channel": "stable",
     "extension_types": ["extension", "legacy_packaged_app"]
   },
   "identity": {
     "channel": "stable",
     "extension_types": ["extension", "platform_app"]
   },
+  "identity.email": {
+    "channel": "dev",
+    "extension_types": ["extension", "platform_app"]
+  },

[not at google - send to devlin, 2013/10/16 16:49:37]

hm. I thought this was just going to be a separate permission (like outside the
identity API)?

I thought of some other places where such a permission would be nice, for
example, the feedback API might only include the email address of the user if
the permission is requested. That's a bad example I think since it's not the UI
the feedback API would end up having (it'd probably just always show the email
and have the user switch it off) but... there may be other places too?

Not sure about this point, but, wdyt?

[Michael Courage, 2013/10/16 20:24:52]

That is what I originally proposed. 
 
I stuck it under identity because chrome.identity is becoming the home of
information about accounts on your profile, so I thought this seemed natural,
even though other APIs also make use of your account.

In the API space, developers very often think they should be doing something
with the Identity API whenever identity is involved, like when they are using
GCM for Chrome. Argument in favor of "identity.email" -> This seems to be where
developers look when they want to deal with identity. Argument against -> Maybe
I'm enhancing their confusion about the boundaries of the Identity API.

Anyway, if you're not super-convinced that "identity.email" is right after
reading this comment, I'll go ahead and change it to the originally proposed
"userEmail" since that's the string everyone else already saw & lgtm'd.

   "identityPrivate": {
     "channel": "stable",
     "extension_types": [
       "legacy_packaged_app"
     ],
     "location": "component"
   },
   "idltest": {
     "channel": "trunk",
     "extension_types": ["extension"]
@@ skipping 434 matching lines @@
     // Only allowed for whitelisted extensions until all the caveats are
     // addressed. Tracked in crbug/285151.
     "channel": "stable",
     "extension_types": ["extension"],
     "location": "component",
     "whitelist": [
       "mfffpogegjflfpflabcdkioaeobkgjik"  // GAIA Component Extension
     ]
   }]
 }