Add mojom category to slow-reports

The mojom category is needed to debug what is happening on the IO/Thread.
We are seeing janks related to communications between renderers, browser
and GPU. We believe the mojom categoy will help us doing investigation.

R=oysteine@chromium.org

Bug: 867931
Change-Id: Ic01d67ca82cf60e949793eaf5ac1a09486452519
Reviewed-on: https://chromium-review.googlesource.com/c/1437065Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626174}

content/browser/tracing/background_tracing_manager_impl.cc

@@ -726,7 +726,7 @@ TraceConfig BackgroundTracingManagerImpl::GetConfigForCategoryPreset(
     case BackgroundTracingConfigImpl::CategoryPreset::BENCHMARK_NAVIGATION: {
       auto config = TraceConfig(
           "benchmark,toplevel,ipc,base,browser,navigation,omnibox,ui,shutdown,"
-          "safe_browsing,Java,EarlyJava,loading,startup,"
+          "safe_browsing,Java,EarlyJava,loading,startup,mojom,"
           "disabled-by-default-system_stats,disabled-by-default-cpu_profiler",
           record_mode);
       // Filter only browser process events.
@@ -737,7 +737,7 @@ TraceConfig BackgroundTracingManagerImpl::GetConfigForCategoryPreset(
     }
     case BackgroundTracingConfigImpl::CategoryPreset::BENCHMARK_RENDERERS:
       return TraceConfig(
-          "benchmark,toplevel,ipc,base,ui,v8,renderer,blink,blink_gc,"
+          "benchmark,toplevel,ipc,base,ui,v8,renderer,blink,blink_gc,mojom,"
           "disabled-by-default-v8.gc,"
           "disabled-by-default-blink_gc,"
           "disabled-by-default-renderer.scheduler,"