Don't explode if --tools or --extra-tools isn't specified.

BUG=none

Review-Url: https://codereview.chromium.org/2618633005
Cr-Commit-Position: refs/heads/master@{#441846}

tools/clang/scripts/update.py

@@ -847,7 +847,7 @@ def main():
                       help='print current clang version (e.g. x.y.z) and exit.')
   parser.add_argument('--run-tests', action='store_true',
                       help='run tests after building; only for local builds')
-  parser.add_argument('--extra-tools', '--tools', nargs='*',
+  parser.add_argument('--extra-tools', '--tools', nargs='*', default=[],
                       help='select additional chrome tools to build')
   parser.add_argument('--without-android', action='store_false',
                       help='don\'t build Android ASan runtime (linux only)',