Adds a couple more measures to the benchmarking app

BUG=
R=ppi@chromium.org

Review URL: https://codereview.chromium.org/1394963002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 0d891ea1f48e9f630d1cd524ba60499b0e5760a9

mojo_benchmark

diff --git a/mojo_benchmark b/mojo_benchmark
index 476b8e3..e0ffff0 100755
--- a/mojo_benchmark
+++ b/mojo_benchmark
@@ -36,6 +36,8 @@
 
   - 'time_until' - time until the first occurence of the targeted event
   - 'avg_duration' - average duration of the targeted event
+  - 'percentile_duration' - value at XXth percentile of the targeted event where
+    XX is from the measurement spec, i.e. .../<event name>/0.XX
 
 |benchmark_list_file| may reference the |target_os| global that will be any of
 ['android', 'linux'], indicating the system on which the benchmarks are to be