Reland of Add a library for running external commands, providing timeouts and test injection.

Reland of Add a library for running external commands, providing timeouts and test injection. (patchset #1 id:1 of https://codereview.chromium.org/1299493004/ )

Reason for revert:
Fixed bug in previous fix - see patchset #2.

Original issue's description:
> Revert of Add a library for running external commands, providing timeouts and test injection. (patchset #2 id:140001 of https://codereview.chromium.org/1302513006/ )
> 
> Reason for revert:
> CT Perf tasks are immediately failing:
> https://uberchromegw.corp.google.com/i/skia-ct-master/run_chromium_perf_on_workers.build101-m5.chrome-bot.log.INFO.20150819-130840.6185
> 
> It is likely unrelated to this CL but I am going to try reverting it and rerunning to see what happens.
> 
> Original issue's description:
> > Reland of Add a library for running external commands, providing timeouts and test injection. (patchset #1 id:1 of https://codereview.chromium.org/1295043002/ )
> > 
> > Reason for revert:
> > Fixed bug in original code - see patchset #2.
> > 
> > Original issue's description:
> > > Revert of Add a library for running external commands, providing timeouts and test injection. (patchset #2 id:40001 of https://codereview.chromium.org/1290913004/ )
> > > 
> > > Reason for revert:
> > > CT Chromium Perf tasks are failing with:
> > > 
> > > E0815 11:07:22.649344   13091 exec.go:193] Command exited with invalid argument: python /b/skia-repo/go/src/go.skia.org/infra/ct/py/sync_skia_in_chrome.py --destination=/b/storage/chromium_builds/linux_base --fetch_target=chromium --chrome_revision=6ae54795f6d077e31f4fb42540ead46f4141f2bb --skia_revision=fb28cd2b13d76c324570341e1155aaadc1b532b6
> > > 
> > > Original issue's description:
> > > > Add a library for running external commands, providing timeouts and test injection.
> > > > 
> > > > BUG=skia:4106
> > > > 
> > > > Committed: https://skia.googlesource.com/buildbot/+/b65fde6749a3e060938cd280c7e281e29516edc1
> > > 
> > > TBR=jcgregorio@google.com,benjaminwagner@google.com
> > > NOPRESUBMIT=true
> > > NOTREECHECKS=true
> > > NOTRY=true
> > > BUG=skia:4106
> > > 
> > > Committed: https://skia.googlesource.com/buildbot/+/ac6c1292955c888d35f6c01cf97d3b6ff4029424
> > 
> > BUG=skia:4106
> > 
> > Committed: https://skia.googlesource.com/buildbot/+/a12e89205d4145b4da14b0a7d5b95b15b029ada9
> 
> TBR=jcgregorio@google.com,benjaminwagner@google.com
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=skia:4106
> 
> Committed: https://skia.googlesource.com/buildbot/+/0a44649b05aa6f864a45268f681f88328f93796d

BUG=skia:4106
Committed: https://skia.googlesource.com/buildbot/+/92ff10b1d8f151c43a3f2ebc50b547a93b74d891

[dogben, 2015-08-20 01:59:28]

Created Reland of Add a library for running external commands, providing
timeouts and test injection.

[dogben, 2015-08-20 02:08:53]

Sorry for the continued breakage. PTYAL

[rmistry, 2015-08-20 15:01:35]

lgtm

[rmistry, 2015-08-20 15:05:35]

The CQ bit was checked by rmistry@google.com

[commit-bot: I haz the power, 2015-08-20 15:05:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1300273002/140001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1300273002/140001

[commit-bot: I haz the power, 2015-08-20 15:07:15]

Committed patchset #2 (id:140001) as
https://skia.googlesource.com/buildbot/+/92ff10b1d8f151c43a3f2ebc50b547a93b74...