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X events have a timestamp which is only well defined relative to the X11 Server
time. The previous computation for timestamp for X11 events was making the
assumption that Server time and Chrome time were the same. This assumption is
not always true -- this is likely the root cause of "bad" timestamps observed in
https://bugs.chromium.org/p/chromium/issues/detail?id=650338#c1

This CL changes event timestamp computation to make a roundtrip to the X11
Server to get an accurate base::TimeTicks. This logic was lifted out of the
responsiveness calculator, which was already doing this computation. The latter
will subsequently be changed to use the computation in this CL.

Change-Id: I963019cd8bfb8ce14e06b3743a159c9c85f2cb82
Bug: 859155
Reviewed-on: https://chromium-review.googlesource.com/1249383Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594844}