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

Revert of Create classes_util API, change discover to return a list instead of a dict. (patchset #1 id:1 of https://codereview.chromium.org/1262623007/)

Reason for revert:
Landed even though the patch failed.

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: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}
>
> TBR=dtu@chromium.org,bengr@chromium.org,avi@chromium.org
> BUG=498968
>
> Committed: https://crrev.com/2c1531af216c026be0b4aed672dc89d264f95a72
> Cr-Commit-Position: refs/heads/master@{#341603}

TBR=dtu@chromium.org,bengr@chromium.org,avi@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=498968

Review URL: https://codereview.chromium.org/1272433002

Cr-Commit-Position: refs/heads/master@{#341621}