Avoid twice clock reading

Avoid twice clock reading

Add TRACE_EVENT_FLAG_EXPLICIT_TIMESTAMP which is set when an explicit
timestamp is specified for the trace event. Only re-read clock if the
flag is set.

BUG=457441
Committed: https://crrev.com/02769c33ee568c5329ce85702428635e2ef442ee
Cr-Commit-Position: refs/heads/master@{#317618}

[Xianzhu, 2015-02-13 20:01:14]

wangxianzhu@chromium.org changed reviewers:
+ dsinclair@chromium.org, nduca@chromium.org

[nduca, 2015-02-17 15:55:20]

lgtm

[Xianzhu, 2015-02-23 17:42:32]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2015-02-23 17:43:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/928563003/1

[commit-bot: I haz the power, 2015-02-23 17:45:37]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-23 17:45:38]

Try jobs failed on following builders:
  ios_dbg_simulator on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[Xianzhu, 2015-02-23 17:49:52]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2015-02-23 17:51:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/928563003/1

[commit-bot: I haz the power, 2015-02-23 19:14:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-23 19:14:34]

Patchset 1 (id:??) landed as
https://crrev.com/02769c33ee568c5329ce85702428635e2ef442ee
Cr-Commit-Position: refs/heads/master@{#317618}