In Windows (already worked correctly in posix): gclient wouldn't correctly update depot_tools when called.

Refactored away the bootstrap depot_tools update flow which only still existed in Windows.

Patch contributed by gab@chromium.org

R=maruel@chromium.org
BUG=
TEST=Running `gclient sync` from src/ and see depot_tools being correctly updated under a git depot_tools checkout.

Review URL: https://chromiumcodereview.appspot.com/9751009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@132191 0039d316-1c4b-4281-b951-d872f2087c98