Use CLOCK_REALTIME_COARSE when available.
Use CLOCK_REALTIME_COARSE when available.

On systems that have CLOCK_REALTIME_COARSE with good enough resolution,
we can avoid making a system call to get the current time; it's serviced
from the vDSO.

This is v2 of the patch.  v1 can be found at [0] but was reverted in [1]
because of Chromium sandbox restrictions.  The necessary changes have
been applied upstream in [2].

[0] https://codereview.chromium.org/1125003002
[1] https://codereview.chromium.org/1130083003
[2] https://codereview.chromium.org/1133653002

BUG=
LOG=N

Review URL: https://codereview.chromium.org/1151283005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#28639}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh