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.

BUG=
LOG=N

Review URL: https://codereview.chromium.org/1125003002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#28280}

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