GitLab

Add plumbing for registration from push service-agnostic content layer
to GCMDriver in chrome layer.

To achieve this, GCMProfileService owns an implementation of a new
PushMessagingService class, which can be used from the content layer.

Based on mvanouwerkerk's prototype in https://codereview.chromium.org/186023002, but significantly refactored.

BUG=350384
TBR=benm@chromium.org

Review URL: https://codereview.chromium.org/317823007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276792 0039d316-1c4b-4281-b951-d872f2087c98