nanobench: fix and simplify --samplingTime

nanobench: fix and simplify --samplingTime

Recent changes to WallTimer broke --samplingTime.  In particular, this idiom became nonsensical:
   WallTimer timer;
   timer.start();
   do {
     ...
     timer.end();
   } while(timer.fWall < ...);

WallTimer started making private use of fWall between when start() and end() were called, so the second time around the loop we end up with nonsense.

If that makes no sense, don't worry.  The code here using now_ms() is just as fast, just as precise, and clearer.

I took the opportunity to simplify --samplingTime <complicated string parsing> to --ms <int>, and to simplify the code that depends on it.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/bbba16878f343b232d844281fbdf056c00e20fb6

[mtklein_C, 2015-10-28 17:55:33]

Description was changed from

==========
Fix and simplify --samplingTime

BUG=skia:
==========

to

==========
nanobench: fix and simplify --samplingTime

Recent changes to WallTimer broke --samplingTime.  In particular, this idiom
became nonsensical:
   WallTimer timer;
   timer.start();
   do {
     ...
     timer.end();
   } while(timer.fWall < ...);

WallTimer started making private use of fWall between when start() and end()
were called, so the second time around the loop we end up with nonsense.

If that makes no sense, don't worry.  The code here using now_ms() is just as
fast, just as precise, and clearer.

I took the opportunity to simplify --samplingTime <complicated string parsing>
to --ms <int>, and to simplify the code that depends on it.

BUG=skia:
==========

[mtklein_C, 2015-10-28 17:58:26]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-28 17:58:32]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1419103004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1419103004/20001

[mtklein_C, 2015-10-28 18:01:33]

mtklein@chromium.org changed reviewers:
+ bsalomon@google.com

[commit-bot: I haz the power, 2015-10-28 18:17:45]

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

[commit-bot: I haz the power, 2015-10-28 18:17:46]

Dry run: This issue passed the CQ dry run.

[bsalomon, 2015-10-28 18:26:54]

On 2015/10/28 18:01:34, mtklein_C wrote:

lgtm

[mtklein_C, 2015-10-28 18:35:49]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-10-28 18:35:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1419103004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1419103004/20001

[commit-bot: I haz the power, 2015-10-28 18:36:32]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/bbba16878f343b232d844281fbdf056c00e20fb6