Convert .ninja_log files to chrome's about:tracing format.

To view traces, either open about:tracing in chrome, or use https://www.speedscope.app/ or https://ui.perfetto.dev/.

Idea from Nick Carter, initial implementation from Richard Smith.

$ ./ninjatracing
Converts one (or several) .ninja_log files into chrome's about:tracing format

Usage:
    ninja -C $BUILDDIR
    ninjatracing $BUILDDIR/.ninja_log > trace.json

By default this will show build timing results for the most recent (possibly
incremental) build. To show build timing results for every target, whether
built in the last build or previously, use --showall. Note that this will
overlap multiple builds and will thus exaggerate build parallelism.

(When using --showall, ideally rm $BUILDDIR/.ninja_log and do a clean build.
If you don't have time for a clean build, at least run
`ninja -C $BUILDDIR -t recompact` first to remove no-longer-built targets.)

The results can be converted from .json format to .html format, for easier
loading into about:tracing, using trace2html from:
https://github.com/catapult-project/catapult/blob/master/tracing/bin/trace2html