[cc/metrics] Report a diagnostic metric to understand delayed frames.

Report the offset between the actual presentation timestamp, and the
expected frame-deadline (i.e. vsync time). We will have this metric for
a shortwhile to validate our assumption that the permissive deadline we
currently use to detect 'missed deadline' frames are correct.

BUG=1064574

Change-Id: Ib5a9c4085deb4140ab037dd897a1a76a65cd67d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2237657
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Behdad Bakhshinategh <behdadb@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777080}