[Benchmarking] Hotfix for gtest OBBS.

Hotfix for https://chromium-review.googlesource.com/c/chromium/src/+/1983326
Failure example: https://chrome-swarming.appspot.com/task?id=49652cd3c145fd10

TBR=johnchen@chromium.org

Bug: 921353
Change-Id: Ia8eef9bc53fbe293a2028a8b228f78a73926ded7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1983464Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727692}

testing/scripts/run_performance_tests.py

@@ -613,9 +613,12 @@ def main(sys_args):
         names_and_configs = shard_configuration['executables']
         for (name, configuration
              ) in names_and_configs.iteritems():
+          additional_flags = []
+          if 'arguments' in configuration:
+            additional_flags = configuration['arguments']
           command_generator = GtestCommandGenerator(
               options, override_executable=configuration['path'],
-              additional_flags=configuration['arguments'])
+              additional_flags=additional_flags)
           output_paths = OutputFilePaths(isolated_out_dir, name).SetUp()
           print('\n### {folder} ###'.format(folder=name))
           return_code = execute_gtest_perf_test(