[telemetry benchmarks wrapper] override test repeat argument if asked to

I checked if you run like this:
./run_benchmark ... --pageset-repeat=3 --pageset-repeat=1
then the second argument overrides the first.

Bug: 894254
Change-Id: Ib7138966dfa5336697a7a5c02084557f315aac4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628161Reviewed-by: Chan Li <chanli@chromium.org>
Commit-Queue: Chan Li <chanli@chromium.org>
Auto-Submit: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662906}

testing/scripts/run_performance_tests.py

@@ -259,12 +259,13 @@ class TelemetryCommandGenerator(object):
     return ([sys.executable, self._options.executable] +
             [self.benchmark] +
             self._generate_filter_args() +
-            self._generate_repeat_args() +
             self._generate_also_run_disabled_tests_args() +
             self._generate_output_args(output_dir) +
             self._generate_story_range_args() +
-            # passthrough args must be before reference args: crbug.com/928928
+            # passthrough args must be before reference args and repeat args:
+            # crbug.com/928928, crbug.com/894254#c78
             self._get_passthrough_args() +
+            self._generate_repeat_args() +
             self._generate_reference_build_args()
            )