Clang update script: start working on the Python version

We currently fall back to update.sh for non-Windows, but eventually
we want the python version to be used on all platforms.

BUG=82385

Review URL: https://codereview.chromium.org/145733003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250356 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250356