post_build_ninja_summary.py analyzes the .ninja_log file in an output
directory to summarize build performance. It lists the most expensive
build steps, and build-step types, printing both their elapsed time and,
more importantly, their "weighted" time which is a measure of their
contribution to the total build time.

For each time segment within the build the running build steps
accumulate weighted time that is proportional to the elapsed time
divided by the number of tasks running. If a thousand compilation steps
are running in parallel then they will each be "charged" for 1/1,000th
of the elapsed time. If a single link step is running alone then it is
charged with all of the elapsed time.

Bug: chromium:787983
Change-Id: Id5aea715f798a16415dd0365a27f0051202668e5
Reviewed-on: https://chromium-review.googlesource.com/871988Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>