Include perfetto tracing instructions

[emilieroberts]

Probably the best way to be tracing on Chrome OS right now.

• Get helper script as indicated in perfetto.dev docs

curl -O https://raw.githubusercontent.com/google/perfetto/master/tools/record_android_trace
chmod u+x record_android_trace

• put device in dev mode
• connect via adb
• use script to grab the trace : "./record_android_trace -o trace_file.perfetto-trace -t 40s -b 128mb sched freq idle am wm gfx view binder_driver hal dalvik camera input res memory" -- t == max length -- b == max size -- determine other flags from perfetto ui or docs
• Read trace file with ui.perfetto.dev

Include some screen shots, links to perfetto.dev.