GitLab

Wires the call to Add/RemoveHeartbeatInterval through:
* GCMDriver/Desktop/Android/Fake
* GCMDriverDesktop::IOWorker
* GCMClient/Impl to MCSClient
* Adding loading of persisted interval in MCSClient

BUG=481174

Review URL: https://codereview.chromium.org/1124783002

Cr-Commit-Position: refs/heads/master@{#329750}