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Sampling profiler uses TimeTicks::Now in signal handler to get a
timestamp associated with the sample.
When called from signal handler, TimeTicks::Now occasionally crashes
the  following check:
CHECK(clock_gettime(clk_id, &ts) == 0)

In order to get good time precision in profiler, we'd like to get
a timestamp while the sampled thread is paused. Since failures are rare,
a solution is to allow clock_gettime to fail, and fallback to getting
an 'async' timestamp after resume.

To reuse some of the ConvertTimespecToMicros boilerplate common with
TimeTicks::Now, this CL exposes
subtle::MaybeTimeTicksNowIgnoringOverride() on posix, used in
stack_coper_signal.


Bug: 1155269
Change-Id: I4d8f5a3e050f5285b87297f13dfd29e888e40257
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584603
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836298}