Add system stats to navigation category in slow-reports

This CL is activating the "system_stats" category for the navigation
slow-reports.

The idea is to collect more system wide information to determine the
cause of janks. We believe many janks are related to system wide
properties not visible without system wide metrics.

R=fdoray,oysteine

Bug: 872900
Change-Id: I2151616b07b79997b20243181eb31c5270c4ee68
Reviewed-on: https://chromium-review.googlesource.com/1171109Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582319}

content/browser/tracing/background_tracing_manager_impl.cc

@@ -653,7 +653,8 @@ BackgroundTracingManagerImpl::GetCategoryFilterStringForCategoryPreset(
         BENCHMARK_EXECUTION_METRIC:
       return "blink.console,v8";
     case BackgroundTracingConfigImpl::CategoryPreset::BENCHMARK_NAVIGATION:
-      return "benchmark,toplevel,ipc,base,browser,navigation,omnibox";
+      return "benchmark,toplevel,ipc,base,browser,navigation,omnibox,"
+             "disabled-by-default-system_stats";
     case BackgroundTracingConfigImpl::CategoryPreset::BLINK_STYLE:
       return "blink_style";
     case BackgroundTracingConfigImpl::CategoryPreset::CATEGORY_PRESET_UNSET: