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This CL uses ui::PlatformEventSource observer API to hook into pre and post
native event dispatch on Linux.

Timestamps for X11 events are in X11 server time. The implementation makes a
round-trip to the X11 server to obtain a delta, and then uses that delta with
base::TimeTicks::Now() to convert to a base::TimeTicks.

The overhead from the rount-trip seems reasonable. 99th percentile X11 RTT is
3ms. 99.9th percentile is 16ms. 99.99th percentile is 40ms. These include the
time for syscalls to base::TimeTicks::Now().

To further reduce overhead, this CL changes the UMA metric to only be computed
with 1/1000 frequency, to avoid two calls to base::TimeTicks::Now().

Note: The implementation of native event observer will be turned on via Finch
experiment to measure overhead.

Change-Id: I786537f093e862a7a19630c9150606a2eb8267d9
Bug: 859155
Reviewed-on: https://chromium-review.googlesource.com/1162658Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581304}