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Now that keys and IDs are associated with projects, not events, the
documentation and method signatures in key_data is out of date.

I've also changed the name of HashForEventId to HmacMetric, since it
was a bit cryptic. Note that HmacMetric still operates on the values of
an event, not a project.

Bug: 1148168
Change-Id: I4bc8e6360917f4b4f141aaad9a11e5485954871f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2554193Reviewed-by: Thanh Nguyen <thanhdng@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830728}