Ignore the return code of the reference build.

Now that the recipe doesn't see each benchmark, we don't want a swarming
shard to fail if just the reference build fails.

Bug: 758630
Change-Id: I67a11a8ef18e9a3a5405896367e67faa542b584f
Reviewed-on: https://chromium-review.googlesource.com/975626Reviewed-by: Ashley Enstad <ashleymarie@chromium.org>
Commit-Queue: Emily Hanley <eyaich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545085}

testing/scripts/run_performance_tests.py

@@ -207,8 +207,9 @@ def main():
     for benchmark in sharding:
       return_code = (execute_benchmark(
           benchmark, isolated_out_dir, args, rest_args, False) or return_code)
-      return_code = (execute_benchmark(
-          benchmark, isolated_out_dir, args, rest_args, True) or return_code)
+      # We ignore the return code of the reference build since we do not
+      # monitor it.
+      execute_benchmark(benchmark, isolated_out_dir, args, rest_args, True)
 
   return return_code