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This API is not yet connected to anything in the net stack. Most of
these new metrics will be plumbed through from net::LoadTimingInfo, and
a few from other places.

Includes a unit test for the big mess of longs/Dates in
RequestFinishedInfo.Metrics, but no other tests yet because nothing
works yet.

Leaves the old APIs in place for now, since those actually work.

BUG=629194

Review-Url: https://codereview.chromium.org/2220023002
Cr-Commit-Position: refs/heads/master@{#419185}