GitLab

Extracting GCMConnectionObserver from GCMAppHandler
* Updating consuming code: GCMInvalidationBridge
* Updating GCMAppHandler overrides
* Updating tests for GCMDriverDesktop

BUG=407841
R=jianli@chromium.org

Review URL: https://codereview.chromium.org/515763002

Cr-Commit-Position: refs/heads/master@{#292396}