This is needed because the current implementation commonly fails on Windows due to "directory not empty" errors.  Using rd is more reliable.

Review URL: https://chromiumcodereview.appspot.com/13581005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@194932 0039d316-1c4b-4281-b951-d872f2087c98