Docserver: Default to master commit ID on refresh

Docserver: Default to master commit ID on refresh

Currently _refresh handler caches data uses tip of tree
and caches data under the remote's master commit ID if
run without a 'commit' paramter.

This CL changes the handler to use the last cached 'master'
commit ID instead, so that refresh tasks for non-chromium-git
data can be run between refresh cycles.

BUG=None
R=kalman@chromium.org
NOTRY=True

Committed: https://crrev.com/10e7c0fd59ec70f46dd0df4f188d34c22960d502
Cr-Commit-Position: refs/heads/master@{#302635}

[Ken Rockot(use gerrit already), 2014-11-04 18:44:05]

ben could you take a look?

[not at google - send to devlin, 2014-11-04 18:47:30]

lgtm

though could you do me a favour and add a bit of logging to track down
https://code.google.com/p/chromium/issues/detail?id=429034 (I couldn't be
bothered given the overhead of pushing a new server version - but if you're
doing it anyway... cool).

Namely, log the base URL here:
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/common/exte...

[Ken Rockot(use gerrit already), 2014-11-04 18:55:44]

done

[Ken Rockot(use gerrit already), 2014-11-04 18:56:03]

The CQ bit was checked by rockot@chromium.org

[commit-bot: I haz the power, 2014-11-04 18:57:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/684113003/40001

[commit-bot: I haz the power, 2014-11-04 18:58:37]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2014-11-04 18:59:13]

Patchset 3 (id:??) landed as
https://crrev.com/10e7c0fd59ec70f46dd0df4f188d34c22960d502
Cr-Commit-Position: refs/heads/master@{#302635}