[test-runner] Enable specification of trybots.

[test-runner] Enable specification of trybots.

Now it's possible to specify the desired trybots for perf
tries, e.g.:

tools/try_perf.py --linux64_haswell octane sunspider

BUG=chromium:478460
LOG=n
NOTRY=true

Committed: https://crrev.com/4b8bb5ed1e3f03f62a0b1da06acaee5bcecc98e7
Cr-Commit-Position: refs/heads/master@{#28142}

[Michael Achenbach, 2015-04-29 11:57:26]

machenbach@chromium.org changed reviewers:
+ bmeurer@chromium.org, mvstanton@chromium.org

[Michael Achenbach, 2015-04-29 11:59:21]

PTAL

[Benedikt Meurer, 2015-04-30 04:18:57]

The CQ bit was checked by bmeurer@chromium.org

[Benedikt Meurer, 2015-04-30 04:18:58]

lgtm

[commit-bot: I haz the power, 2015-04-30 04:19:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1114913002/1

[commit-bot: I haz the power, 2015-04-30 05:12:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-30 05:12:59]

Patchset 1 (id:??) landed as
https://crrev.com/4b8bb5ed1e3f03f62a0b1da06acaee5bcecc98e7
Cr-Commit-Position: refs/heads/master@{#28142}