Fix flaky TabListBackendTest

Fix flaky TabListBackendTest

The tests should only run when tab control is supported.

See log on FYI bot Android Tests (K N5, user build)(dbg):
http://build.chromium.org/p/chromium.fyi/builders/Android%20Tests%20%28K%20N5%2C%20user%20build%29%28dbg%29/builds/3050/steps/telemetry_unittests/logs/stdio

BUG=437881
Committed: https://crrev.com/fcfd80de79aa6bf4344be9e611ff988a6312a1f3
Cr-Commit-Position: refs/heads/master@{#319103}

[Zhen Wang, 2015-03-04 16:53:44]

zhenw@chromium.org changed reviewers:
+ erikchen@chromium.org, nednguyen@google.com

[Zhen Wang, 2015-03-04 16:53:45]

ptal

[sullivan, 2015-03-04 16:56:49]

sullivan@chromium.org changed reviewers:
+ sullivan@chromium.org

[sullivan, 2015-03-04 16:56:49]

lgtm

(stealing this since Ned is super busy this week)

[Zhen Wang, 2015-03-04 17:17:52]

The CQ bit was checked by zhenw@chromium.org

[commit-bot: I haz the power, 2015-03-04 17:19:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/979833003/1

[commit-bot: I haz the power, 2015-03-04 17:28:49]

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

[commit-bot: I haz the power, 2015-03-04 17:28:50]

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

[Zhen Wang, 2015-03-04 19:09:15]

The CQ bit was checked by zhenw@chromium.org

[commit-bot: I haz the power, 2015-03-04 19:14:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/979833003/1

[commit-bot: I haz the power, 2015-03-04 19:44:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 19:45:20]

Patchset 1 (id:??) landed as
https://crrev.com/fcfd80de79aa6bf4344be9e611ff988a6312a1f3
Cr-Commit-Position: refs/heads/master@{#319103}