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 #2 id:280001 of https://codereview.chromium.org/1262623007/ )

Reason for revert:
fails reliably on XP tests: https://build.chromium.org/p/chromium.win/builders/XP%20Tests%20(1)/builds/39209/steps/telemetry_perf_unittests/logs/stdio

Original issue's description:
> Reland of Create classes_util API, change discover to return a list instead of a dict. (patchset #1 id:1 of https://codereview.chromium.org/1263063003/)
>
> Reason for revert:
> 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.
>
> Original issue's description:
> > 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: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}
>
> TBR=dtu@chromium.org,bengr@chromium.org,avi@chromium.org
> 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/7e6fefcf3b839256c97e2e7bc0bfc3c53af75c8d
> Cr-Commit-Position: refs/heads/master@{#341646}

TBR=dtu@chromium.org,bengr@chromium.org,avi@chromium.org,aiolos@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=498968
Committed: https://crrev.com/bf85d83ff178f0ce51e4f91641ab7887397a5e20
Cr-Commit-Position: refs/heads/master@{#341692}

[jochen (gone - plz use gerrit), 2015-08-04 07:03:45]

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

[commit-bot: I haz the power, 2015-08-04 07:04:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1267243002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1267243002/1

[commit-bot: I haz the power, 2015-08-04 07:06:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-04 07:07:13]

Patchset 1 (id:??) landed as
https://crrev.com/bf85d83ff178f0ce51e4f91641ab7887397a5e20
Cr-Commit-Position: refs/heads/master@{#341692}