Systrace is now part of Catapult project. The catapult directory contains code
from:

    https://github.com/catapult-project/catapult.git

The file UPSTREAM_REVISION contains the upstream revision number that was last
pulled.  Changes should be made upstream and then pulled into the catapult
directory.

The update.py script should be used to pull the latest upstream code, update
the UPSTREAM_REVISION file, and generate systrace_trace_viewer.html. The
generated file is checked into the git repository, and they get embedded in each
trace HTML file that systrace.py generates.