Docserver move cron jobs to a backend instance

Docserver move cron jobs to a backend instance

backends.yaml now specifies a 'worker' backend that will sync with
src.chromium.org and render all the pages. cron.yaml now targets
'worker', which may or may not actually work.

There isn't documentation on this, see [1], but setting target to
the backend name *should* work. It didn't seem to work on the dev server
though.

Removed the logic in cron_servlet.py that dealt with
DeadlineExceededError.

[1] https://code.google.com/p/googleappengine/issues/detail?id=6301

BUG=270808

[方觉(Fang Jue), 2013-08-14 01:47:42]

https://codereview.chromium.org/23081003/diff/1/chrome/common/extensions/docs...
File chrome/common/extensions/docs/server2/appengine_wrappers.py (right):

https://codereview.chromium.org/23081003/diff/1/chrome/common/extensions/docs...
chrome/common/extensions/docs/server2/appengine_wrappers.py:13: #
backend_name:2-0-25.23982345. We only want the 2-0-25.
Backend's version ID will look like <backend-name>.123456. A backend is actually
a separate version of the app (which means that update_server.py also needs to
be updated to also deploy the backend). Maybe include the current version number
in the backend name or pass it in /_cron/ URL?

https://codereview.chromium.org/23081003/diff/1/chrome/common/extensions/docs...
File chrome/common/extensions/docs/server2/cron_servlet.py (left):

https://codereview.chromium.org/23081003/diff/1/chrome/common/extensions/docs...
chrome/common/extensions/docs/server2/cron_servlet.py:11: GetAppVersion,
DeadlineExceededError, IsDevServer, logservice)
DeadlineExceededError can still happen even if it's a backend and there're more
than one type of DeadlineExceededError. See
https://codereview.chromium.org/22457003/diff/22001/chrome/common/extensions/....