Add presubmit trybuilder to Luci-GAE repository.

Add presubmit trybuilder to Luci-GAE repository.

This changes the infra/luci_gae recipe to run presubmit with pcg properly,
and adds a new builder to master.

TEST=locally with https://codereview.chromium.org/1345303002/
R=vadimsh@chromium.org
BUG=529936
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=296720

[tandrii_google, 2015-09-16 20:54:09]

tandrii@google.com changed reviewers:
+ tandrii@google.com

[tandrii_google, 2015-09-16 20:54:10]

PTAL

served best with secret souce https://codereview.chromium.org/1345303002/

[Vadim Sh., 2015-09-16 21:03:40]

lgtm

copy-pasta :-/

[tandrii_google, 2015-09-16 21:17:02]

The CQ bit was checked by tandrii@google.com

[commit-bot: I haz the power, 2015-09-16 21:17:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1350923002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1350923002/1

[commit-bot: I haz the power, 2015-09-16 22:03:27]

Committed patchset #1 (id:1) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=296720