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

Committed: https://crrev.com/28cea2b749f24ba33e6e0c8e343dd0d6258ee302
Cr-Commit-Position: refs/heads/master@{#28639}

[bnoordhuis, 2015-05-25 15:32:35]

ben@strongloop.com changed reviewers:
+ jochen@chromium.org

[bnoordhuis, 2015-05-25 15:32:36]

I included the suggestion from the original CL to use NoBarrier_Load() and
NoBarrier_Store().

It's perhaps a little incongruent because there is still (OS X-specific) code in
time.cc that uses unchecked loads and stores.  I can clean that up in another CL
if you think that's worthwhile.

[jochen (gone - plz use gerrit), 2015-05-26 15:45:06]

jochen@chromium.org changed reviewers:
+ bmeurer@chromium.org

[jochen (gone - plz use gerrit), 2015-05-26 15:45:07]

+bmeurer

[Benedikt Meurer, 2015-05-26 17:43:02]

The CQ bit was checked by bmeurer@chromium.org

[Benedikt Meurer, 2015-05-26 17:43:03]

lgtm

[commit-bot: I haz the power, 2015-05-26 17:43:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1151283005/1

[commit-bot: I haz the power, 2015-05-26 18:10:39]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-26 18:10:54]

Patchset 1 (id:??) landed as
https://crrev.com/28cea2b749f24ba33e6e0c8e343dd0d6258ee302
Cr-Commit-Position: refs/heads/master@{#28639}

[Michael Achenbach, 2015-05-27 14:05:24]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1154783003/ by machenbach@chromium.org.

The reason for reverting is: [Sheriff] This leads to several failures in
chromium and blocks our roll:
https://codereview.chromium.org/1154363002/

Bisect (https://codereview.chromium.org/1152553004/) points to this CL.

Please add the failing chromium trybot on a reland of this CL..