Add thread locking around GetCachePath().

Add thread locking around GetCachePath().

BUG=406662
R=cmp@chromium.org, agable@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291569

[cmp, 2014-08-23 01:47:11]

lgtm

[cmp, 2014-08-23 01:47:45]

The CQ bit was checked by cmp@chromium.org

[commit-bot: I haz the power, 2014-08-23 01:47:58]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/szager@chromium.org/469073004/1

[cmp, 2014-08-23 01:50:38]

The CQ bit was checked by cmp@chromium.org

[szager1, 2014-08-23 01:58:01]

Committed patchset #2 manually as 291569.

[commit-bot: I haz the power, 2014-08-23 04:39:06]

Presubmit check for 469073004-1 failed and returned exit status -2001.
The presubmit check was hung. It took 10262.7 seconds to execute and the time
limit is 900.0 seconds.

Running presubmit commit checks ...
Process PoolWorker-9:
Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python2.7/multiprocessing/pool.py", line 99, in worker
    put((job, i, result))
  File "/usr/lib/python2.7/multiprocessing/queues.py", line 390, in put
    return send(obj)
IOError: [Errno 32] Broken pipe