Add stats from ninja subprocesses to build.trace.gz

Propagate the subprocess stats from ninja to build.trace.gz.

There is too much data here to put into separate counters for each
statistic, so put them into args on each duration event for now.

We would like to track max RSS for each subprocess, but the Linux
kernel inherits the max RSS of the ninja process in each subprocess,
which sets a lower bound on the measurable max RSS to the size of
the ninja process.  The ninja process is large due to the multi-GB
build.ninja files.

Bug: 170701554
Test: examine build.trace.gz
Change-Id: I8aaaafe627a57f1a500af098c097c6381c583ba5