Clear GCM data when the user clears cookies and other site data

This is needed for the work to drop GCM sign-in enforcement. Since
we no longer wipe out the GCM data when the user signs out, we need
to give user a chance to clear the data if he or she wants.

TEST=Manual test by selecting "Cookies and other site and plugin data"

Review URL:

Cr-Commit-Position: refs/heads/master@{#294754}
12 files changed