This is in preparation for once again making 1.8.5.2 the default Windows git.

Also remove the CHROME_HEADLESS condition, which confuses bot setup because the
same bot will sometimes run without that set (e.g. at slave startup) and with
it set (e.g. during a build), leading to the wrong, or even no, git installed.

R=maruel@chromium.org

Review URL: https://codereview.chromium.org/160033002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@250486 0039d316-1c4b-4281-b951-d872f2087c98