[GCM] Updating the list of restricted keys for GCM API messages

* ensuring that "goog." and "google" are case-insensitive
* adding "collapse_key" (case-sensitive)
* fixing a typo in error messages in gcm_api.cc and tests: occured -> occurred.

BUG=316683

Review URL: https://codereview.chromium.org/141433030

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250868 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250868