Adds limited cpu time metric to system health benchmarks

We are experimentally enabling some of the histograms of the new cpu
time metric on the perf dashboard so we can evaluate the noise level and
discover any other existing issues of the metric.

Bug=catapult:#3325

Change-Id: I292d7228bde22dafa7f2daf1689896a0729b2631
Reviewed-on: https://chromium-review.googlesource.com/809202Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Deepanjan Roy <dproy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521995}

tools/perf/benchmarks/system_health.py

@@ -50,6 +50,7 @@ class _CommonSystemHealthBenchmark(perf_benchmark.PerfBenchmark):
         'powerMetric',
         'tracingMetric',
         'accessibilityMetric',
+        'limitedCpuTimeMetric'
     ])
     loading_metrics_category.AugmentOptionsForLoadingMetrics(options)
     # The EQT metric depends on the same categories as the loading metric.