Change run-swarmed.py to not build by default.

You can pass --build if you want the script to build before swarming.

See "Should run-swarmed.py default to building?" on chromium-dev.

Bug: none
Change-Id: Id80d78670612f77502a03d80759cc3788200de87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050783
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740642}

tools/run-swarmed.py

@@ -169,9 +169,6 @@ def main():
   parser.add_argument('out_dir', type=str, help='Build directory.')
   parser.add_argument('target_name', type=str, help='Name of target to run.')
 
-  # TODO(thakis): Should this default to false?
-  parser.set_defaults(build=True)
-
   args = parser.parse_args()
 
   if args.target_os == 'detect':