If we modify the startup.mobile benchmark like this:
diff --git a/tools/perf/benchmarks/startup_mobile.py b/tools/perf/benchmarks/startup_mobile.py
index c6e3257a8e14..ada55664025f 100644
--- a/tools/perf/benchmarks/startup_mobile.py
+++ b/tools/perf/benchmarks/startup_mobile.py
@@ -175,6 +175,7 @@ class MobileStartupBenchmark(perf_benchmark.PerfBenchmark):
 
     options = timeline_based_measurement.Options(cat_filter)
     options.config.enable_chrome_trace = True
+    options.config.enable_atrace_trace = True
     options.SetTimelineBasedMetrics([
         'tracingMetric',
         'androidStartupMetric',

Then we would get a trace like the attached one.

Before the Metrics tab (and other side tabs) are loaded, we hit an exception. As seen from trace viewer in catapult:
Uncaught (in promise) TypeError: Cannot read property 'userFriendlyName' of undefined
    at HTMLElement.updateProcessList_ (timeline_view.html:392)
    at HTMLElement.updateContents_ (timeline_view.html:520)
    at HTMLElement.build (timeline_view.html:443)
    at HTMLElement.set model [as model] (timeline_view.html:435)
    at trace_viewer.html:90

Ned noticed that it is likely because the trace has rows that do not correspond to threads/processes. For example, there is a CPU breakdown (meta-row called "Kernel"). I'll upload Ned's suggested fix shortly.
 

Deleted: startup_mobile_atrace.html.gz 6.4 MB

startup_mobile_atrace.html.gz 6.4 MB Download