Add support for CHROMIUM_OUT_DIR to findbugs util.

Add support for CHROMIUM_OUT_DIR to findbugs util.

Currently, the findbugs_diff.py and friends ignore the
CHROMIUM_OUT_DIR environment variable, and hardcodes the out folder
to just be 'out'. This is problematic for developers who use a
different out folder.

This CL makes the generic findbugs util library read the environment
variable, and then default to 'out' if it is not set.

BUG=None

Committed: https://crrev.com/bb78a46883c8b9a3201b6e2e457dda93de8b7072
Cr-Commit-Position: refs/heads/master@{#293990}

[nyquist, 2014-09-09 17:11:02]

nyquist@chromium.org changed reviewers:
+ cjhopman@chromium.org

[nyquist, 2014-09-09 17:47:07]

nyquist@chromium.org changed reviewers:
+ aurimas@chromium.org
- cjhopman@chromium.org

[nyquist, 2014-09-09 17:47:15]

aurimas: PTAL

[aurimas (slooooooooow), 2014-09-09 18:17:16]

lgtm

[nyquist, 2014-09-09 18:18:09]

The CQ bit was checked by nyquist@chromium.org

[commit-bot: I haz the power, 2014-09-09 18:21:28]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/nyquist@chromium.org/553783007/1

[commit-bot: I haz the power, 2014-09-09 19:47:03]

Committed patchset #1 (id:1) as aedc5f51a21f5df027c0bbd99f8f305ac0575d1c

[commit-bot: I haz the power, 2014-09-10 03:55:02]

Patchset 1 (id:??) landed as
https://crrev.com/bb78a46883c8b9a3201b6e2e457dda93de8b7072
Cr-Commit-Position: refs/heads/master@{#293990}