tools/perf: Experimentally add a TBMv3 metric to system health

On bots where --experimental-tbmv3-metric is not enabled, this metric
does nothing. That flag is only enabled on FYI bots at the moment, and
we want to run system health benchmark with both TBMv2 and v3 metrics
enabled there to monitor for stability.

Bug: 1012687
Change-Id: I38ab894ab5ba6a3ca40689d6f3710c6d72c5ed6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316331Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Deep Roy <dproy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792040}

tools/perf/benchmarks/system_health.py

@@ -59,6 +59,9 @@ class _CommonSystemHealthBenchmark(perf_benchmark.PerfBenchmark):
         'reportedByPageMetric',
         'tracingMetric',
         'umaMetric',
+        # Unless --experimentatil-tbmv3-metric flag is used, this metric does
+        # nothing.
+        'tbmv3:cpu_time_metric',
     ])
     loading_metrics_category.AugmentOptionsForLoadingMetrics(options)
     # The EQT metric depends on the same categories as the loading metric.