[Telemetry] Fix race in session restore measurement.

[Telemetry] Fix race in session restore measurement.

The previous code made it possible to try to access tabs prior to the session
restore completing. This should make the benchmark much more deterministic.

BUG=375979
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=275509

[dtu, 2014-06-04 21:19:13]

lgtm

Is it possible to have the same race in other benchmarks, too, and we've just
never encountered it because Chrome loads faster without a session restore?

[tonyg, 2014-06-04 21:21:45]

On 2014/06/04 21:19:13, dtu wrote:
> lgtm
> 
> Is it possible to have the same race in other benchmarks, too, and we've just
> never encountered it because Chrome loads faster without a session restore?

I don't think so. I think the problem was that this opens multiple tabs and we
were accessing tabs before they were created.

[jeremy, 2014-06-05 10:39:04]

LGTM++ !

https://codereview.chromium.org/313923003/diff/1/tools/perf/measurements/sess...
File tools/perf/measurements/session_restore.py (right):

https://codereview.chromium.org/313923003/diff/1/tools/perf/measurements/sess...
tools/perf/measurements/session_restore.py:40: browser.foreground_tab),
Very nice!!!

[tonyg, 2014-06-05 16:55:53]

The CQ bit was checked by tonyg@chromium.org

[commit-bot: I haz the power, 2014-06-05 16:57:10]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tonyg@chromium.org/313923003/1

[commit-bot: I haz the power, 2014-06-05 18:44:38]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel/bu...)
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)
  win_chromium_x64_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_x64_rel/...)

[commit-bot: I haz the power, 2014-06-05 19:20:43]

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

[commit-bot: I haz the power, 2014-06-05 19:20:44]

Try jobs failed on following builders:
  linux_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel/bu...)

[tonyg, 2014-06-06 17:02:22]

The CQ bit was checked by tonyg@chromium.org

[commit-bot: I haz the power, 2014-06-06 17:03:43]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tonyg@chromium.org/313923003/1

[commit-bot: I haz the power, 2014-06-06 20:22:51]

Change committed as 275509