Revert of Raise exception when devices fail to list tests.

Revert of Raise exception when devices fail to list tests. (patchset #2 id:20001 of https://codereview.chromium.org/1915273002/ )

Reason for revert:
Looks like it is breaking upstream android bots
https://build.chromium.org/p/chromium.linux/builders/Android%20Tests%20%28dbg%29/builds/34054/steps/android_webview_unittests/logs/stdio

Original issue's description:
> Raise exception when devices fail to list tests.
> 
> Before we only make sure that list of test cases are not None, but then
> we realise that list of test cases should not be empty too, since
> sometimes when devices fail to list tests, they will not return None,
> but will return an empty list.
> 
> BUG=585585
> 
> Committed: https://crrev.com/3d73e44f7177fe1dbb6e868120b985a6c9fb8a30
> Cr-Commit-Position: refs/heads/master@{#389576}

TBR=jbudorick@chromium.org,hzl@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=585585

[mikecase (-- gone --), 2016-04-26 19:58:18]

Created Revert of Raise exception when devices fail to list tests.

[commit-bot: I haz the power, 2016-04-26 19:58:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1923053002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1923053002/1

[jbudorick, 2016-04-26 19:59:14]

The CQ bit was unchecked by jbudorick@chromium.org