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This CL changes the account reconcilor to use the new cookie store
API that allows clients to adds callback for cookie changed events.
It adds logic to the chrome sign-in client to register for cookie
changed events on the IO thread and call the cookie changed callback
on the main thread.

Note that in order to access the cookie store, we need to use the
URL context which can only be used on the IO thread. Subscribing for
cookie change events requires thus a thread jump to the IO thread on
all platforms.

BUG=NONE

Review URL: https://codereview.chromium.org/695553002

Cr-Commit-Position: refs/heads/master@{#302639}