GitLab

* Making sure GCMAccountTracker is only depenedent on the GCMDriver
* Replacing GCMDesktopDriver.SetAccountsForCheckin with
  virtual GCMDesktop.SetAccountTokens
* Producing a vector of AccountTokenInfo in GCMAccountTracker

BUG=374969
R=jianli@chromium.org

Review URL: https://codereview.chromium.org/594383003

Cr-Commit-Position: refs/heads/master@{#297190}