Flip GN bots to be purely driven by //testing/buildbot/*.json files for tests.

Flip GN bots to be purely driven by //testing/buildbot/*.json files for tests.

This change removes the legacy hooks for running scripts like 'gn check'
on the GN bots; now what we run is purely driven by the
//testing/buildbot/*.json files in the Chromium repo.

I think this is the last major discrepancy between the chromium_gn
recipe and the chromium and chromium_trybot recipes, so the next
step is to start flipping some of the GN bots over to the
chromium and chromium_trybot recipes (which will require master
restarts).

R=phajdan.jr@chromium.org
BUG=481693
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=295535

[jam, 2015-06-04 17:19:33]

jam@chromium.org changed reviewers:
+ jam@chromium.org

[jam, 2015-06-04 17:19:34]

lgtm

[Dirk Pranke, 2015-06-04 21:02:53]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-06-04 21:04:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1153933004/20001

[commit-bot: I haz the power, 2015-06-04 21:09:27]

Committed patchset #2 (id:20001) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=295535