And for now, defaults to Express.

Main changes:
- Valid hash becomes a set of hashes one for Pro, one for Express
- Include WDK to get an old copy of ATL as that doesn't come
  with Express

BUG=323300
R=maruel@chromium.org

Review URL: https://codereview.chromium.org/148453008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@248622 0039d316-1c4b-4281-b951-d872f2087c98