Fix type error in bb_device_steps that causes test_report to crash

Fix type error in bb_device_steps that causes test_report to crash

Apparently ~15 months ago I introduced a bug that causes the
'test_report' step to crash on the chromium.webkit WebKit Android Nexus 4
bot to crash if more than 10 tests fail, flake, or pass unexpectedly,
and no one has noticed or at least dealt with it until now :(.

TBR=jbudodrick@chromium.org
BUG=

Committed: https://crrev.com/d7cae31462097c866ebd25647cfc09b1ad94ad4e
Cr-Commit-Position: refs/heads/master@{#315222}

[Dirk Pranke, 2015-02-07 23:36:45]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-07 23:37:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/891473007/1

[commit-bot: I haz the power, 2015-02-08 01:05:08]

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

[commit-bot: I haz the power, 2015-02-08 01:05:09]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[Dirk Pranke, 2015-02-08 01:39:32]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-08 01:40:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/891473007/1

[commit-bot: I haz the power, 2015-02-08 02:41:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-08 02:41:41]

Patchset 1 (id:??) landed as
https://crrev.com/d7cae31462097c866ebd25647cfc09b1ad94ad4e
Cr-Commit-Position: refs/heads/master@{#315222}