Peak Gpu Memory Ablation Benchmark Metric

In catapult a new metric is being added to measure the time spent doing
memory ablation.
  https://chromium-review.googlesource.com/c/catapult/+/2187206

We previously enabled the tracing categories needed to enable this metric
  https://chromium-review.googlesource.com/c/chromium/src/+/2246931

This change adds the new metric to rendering and loading benchmarks.

Bug: 908462
Change-Id: Id7cc2bce51a002733d9419d3ef3b6e1969fb25a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2259487Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781924}

tools/perf/benchmarks/loading_metrics_category.py

@@ -31,4 +31,5 @@ def AugmentOptionsForLoadingMetrics(tbm_options):
   cat_filter.AddDisabledByDefault('disabled-by-default-network')
 
   tbm_options.AddTimelineBasedMetric('loadingMetric')
+  tbm_options.AddTimelineBasedMetric('memoryAblationMetric')
   return tbm_options

tools/perf/benchmarks/rendering.py

@@ -72,7 +72,7 @@ class _RenderingBenchmark(perf_benchmark.PerfBenchmark):
     options = timeline_based_measurement.Options(category_filter)
     options.config.chrome_trace_config.EnableUMAHistograms(
         *RENDERING_BENCHMARK_UMA)
-    options.SetTimelineBasedMetrics(['renderingMetric', 'umaMetric'])
+    options.SetTimelineBasedMetrics(['renderingMetric', 'umaMetric', 'memoryAblationMetric'])
     return options