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Add understats metrics for first contentful
paint (FCP) broken down by page scheme.
This CL adds metrics that record if FCP value
was less than certain threshold or not.

For background infomation on understats see
slide 20 of http://shortn/_d9oDk4unEX.

Bug: 883445
Change-Id: I7a657960755c055ab1f2bcdbcf6f9754e78ea8a8
Reviewed-on: https://chromium-review.googlesource.com/c/1328230
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607926}