svn propdel svn:executable shouldn't be called on it.

R=petermayo@chromium.org
BUG=150960


Review URL: https://chromiumcodereview.appspot.com/10962038

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@158038 0039d316-1c4b-4281-b951-d872f2087c98