Async Cookies: Add UMA counters for the API.

cookieStore methods receive individual UMA counters. The getters for
CookieChangeEvent and ExtendableCookieChangeEvent also receive UMA
counters, mostly so that we can count apps that call
subscribeToChanges() when a service worker installs, and then consume
the 'cookiechange' events, without calling another cookieStore method
ever again.

TBR=kinuko

Bug: 729800
Change-Id: I0c2aa37700cb6af7e1dc16b695e40dbffb33688f
Reviewed-on: https://chromium-review.googlesource.com/1060624
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559141}
6 files changed