[GCM] Fetching OAuth2 tokens periodically in account tracker

Relanding patch 631343002 with a fix to the Chrome crashing problem.

BUG=374969
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Review URL: https://codereview.chromium.org/739763002

Cr-Commit-Position: refs/heads/master@{#304729}
7 files changed