Adds a couple more measures to the benchmarking app

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/interfaces/tracing.mojom).
 
 ## Arguments
 
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    `trace_event_category`.
  - `time_between/<first_event_category>/<first_event_name>/<second_event_category>/<second_event_name>`
    measures time between the first occurence of the event named
    `<first_event_name>` in category `<first_event_category>` and the first occurence
    of the event named `<second_event_name>` in category
    `<second_event_category>`. The measurement will fail if the first occurence
    of the first event happens after the first occurence of the second event.
  - `avg_duration/<trace_event_category>/<trace_event_name>` - measures average
    duration of all events named `trace_event_name` in category
    `trace_event_category`.
+ - `percentile_duration/<trace_event_category>/<trace_event_name>/0.XX` -
+   measures the value at the XXth percentile of all events named
+   `trace_event_name` in category `trace_event_category`. E.g.
+   `.../<trace_event_name/0.50` will give the 50th percentile.
 
 ## 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 **Android** and **Linux**.