Added a gitiles-based git poller.
Added a gitiles-based git poller.

Also, a boilerplate unit test for gerrit_agent.py.

BUG=

Review URL: https://codereview.chromium.org/281053002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@274933 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build