Add UI category to track fonts loading

We want to add the "ui" category to trace fonts loading:

  void ShapeRunWithFont(const ShapeRunWithFontInput& in,
                        TextRunHarfBuzz::ShapeOutput* out) {
    TRACE_EVENT0("ui", "RenderTextHarfBuzz::ShapeRunWithFontInternal");

R=oysteine@chromium.org

Bug: 867931
Change-Id: I3d3d07516326b3320428d50d164c7fd6a5c3a90a
Reviewed-on: https://chromium-review.googlesource.com/1252531Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595386}

content/browser/tracing/background_tracing_manager_impl.cc

@@ -673,7 +673,7 @@ TraceConfig BackgroundTracingManagerImpl::GetConfigForCategoryPreset(
       return TraceConfig("blink.console,v8", record_mode);
     case BackgroundTracingConfigImpl::CategoryPreset::BENCHMARK_NAVIGATION: {
       auto config = TraceConfig(
-          "benchmark,toplevel,ipc,base,browser,navigation,omnibox,"
+          "benchmark,toplevel,ipc,base,browser,navigation,omnibox,ui,"
           "safe_browsing,task_scheduler,"
           "disabled-by-default-task_scheduler_diagnostics,"
           "disabled-by-default-system_stats,disabled-by-default-cpu_profiler",