Teach `benchmark.mojo` to save the collected trace file to disk.

apps/benchmark/README.md

 # benchmark
 
 This application connects to another mojo application, collects traces during
 indicated period of time and computes a number of results based on the collected
 traces. It can be used to measure performance of a mojo app, provided that the
 app being benchmarked participates in the [tracing
 ecosystem](../../mojo/services/tracing/public/interfaces/tracing.mojom).
 
 ## Arguments
 
-The benchmarking app expects the following arguments:
+The benchmarking app **requires** the following arguments:
 
  - `--app=<app_url>` - url of the application to be benchmarked
  - `--duration=<duration_seconds>` - duration of the benchmark in seconds
 
-any other arguments are assumed to be descriptions of measurements to be
+The following arguments are **optional**:
+
+ - `--trace-output=<output_file_path>` - local file path at which the collected trace
+   will be written
+
+Any other arguments are assumed to be descriptions of measurements to be
 conducted on the collected trace data. Each measurement has to be of form:
 `<measurement_type>/<trace_event_category>/<trace_event_name>`.
 
 The following measurement types are available:
 
  - `time_until` - measures time until the first occurence of the event named
    `trace_event_name` in category `trace_event_category`.
  - `avg_duration` - measures average duration of all events named
    `trace_event_name` in category `trace_event_category`.
 
 ## Runner script
 
 Devtools offers [a helper script](../../mojo/devtools/common/mojo_benchmark)
 allowing to run a list of benchmarks in controlled caching conditions, both
-on Linux and Android.
+on **Android** and **Linux**.