Allow coarse clocks in clock_get{res,time} calls.

Whitelist CLOCK_MONOTONIC_COARSE and CLOCK_REALTIME_COARSE so we can improve
the performance of Date.now() in V8.

See https://codereview.chromium.org/1125003002/ for background.

BUG=

Review URL: https://codereview.chromium.org/1133653002

Cr-Commit-Position: refs/heads/master@{#331201}
6 files changed