Revert of Create classes_util API, change discover to return a list instead of a dict.

Revert of Create classes_util API, change discover to return a list instead of a dict. (patchset #5 id:120001 of https://codereview.chromium.org/1244223002/)

Reason for revert:
This appears to break the XP telemetry tests. Details on the bug.

Original issue's description:
> Create classes_util API, change discover to return a list instead of a dict.
>
> BUG=498968
> CQ_EXTRA_TRYBOTS=tryserver.chromium.perf:linux_perf_bisect;tryserver.chromium.perf:mac_perf_bisect;tryserver.chromium.perf:win_perf_bisect;tryserver.chromium.perf:android_nexus5_perf_bisect
>
> Committed: https://crrev.com/e6cbec4747aa7fe3c96b1cdb89de21ae77b30ab0
> Cr-Commit-Position: refs/heads/master@{#341129}

TBR=dtu@chromium.org,bengr@chromium.org,aiolos@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=498968
Committed: https://crrev.com/c85e3e1af7e170d5f8ec2b012df6e9337d2352bb
Cr-Commit-Position: refs/heads/master@{#341218}

[Avi (use Gerrit), 2015-07-30 23:04:47]

Created Revert of Create classes_util API, change discover to return a list
instead of a dict.

[commit-bot: I haz the power, 2015-07-30 23:04:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1263063003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1263063003/1

[commit-bot: I haz the power, 2015-07-30 23:09:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-30 23:11:00]

Patchset 1 (id:??) landed as
https://crrev.com/c85e3e1af7e170d5f8ec2b012df6e9337d2352bb
Cr-Commit-Position: refs/heads/master@{#341218}

[aiolos (Not reviewing), 2015-07-30 23:34:14]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1262623007/ by aiolos@chromium.org.

The reason for reverting is: The telemetry tests aren't broken. The
perf_unittests are because they don't pick tests in a deterministic way.
Reverting after a change to make them deterministic..