run-swarmed: Support --repeat for better efficiency

When running a small set of tests, the proportion of time running the
test can be very low. Both "--copies N" and "--copies N/M --repeat M"
should give the same results, but the later can be more cost effective.

Bug: None
Change-Id: I0bfa07f80d2b8f2985cd15153d355bd0aedf2dd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2206221
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776793}

tools/run-swarmed.py

@@ -139,6 +139,8 @@ def _Spawn(args):
     ]
   if args.gtest_filter:
     trigger_args.append('--gtest_filter=' + args.gtest_filter)
+  if args.repeat:
+    trigger_args.append('--repeat=' + args.repeat)
   elif args.target_os == 'fuchsia':
     filter_file = \
         'testing/buildbot/filters/fuchsia.' + args.target_name + '.filter'
@@ -212,6 +214,8 @@ def main():
   parser.add_argument('--gtest_filter',
                       help='Use the given gtest_filter, rather than the '
                            'default filter file, if any.')
+  parser.add_argument(
+      '--repeat', help='Number of times to repeat the specified set of tests.')
   parser.add_argument('--no-test-flags', action='store_true',
                       help='Do not add --test-launcher-summary-output and '
                            '--system-log-file flags to the comment.')