benchmark.mojo: add time_between measurement type.

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 **requires** the following arguments:
 
  - `--app=<app_url>` - url of the application to be benchmarked
  - `--duration=<duration_seconds>` - duration of the benchmark in seconds
 
 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>`.
+`<measurement_type>/<measurement_arg_1>/<measurement_arg_2>/...`.
 
 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`.
+ - `time_until/<trace_event_category>/<trace_event_name>` - measures time until
+   the first occurence of the event named `trace_event_name` in category
+   `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`.
 
 ## 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**.