ok, basic tracing support

This doesn't do anything in the default process-per-task mode, because
those child tasks exit using _exit(), which doesn't trigger the event
tracer destructor to flush.

I don't remember exactly why I exit with _exit(), so I'm going to have
to follow up on that.  But written this way as I think I'm at least
initializing the tracing in the right place for each process for the
future.

In threaded (-j -1) and serial (-j 0) modes, everything seems to work
great.

I'm also thinking I might add a tracer like the SkDebugf tracer but
using ok_log(), which handles interlaced logging from concurrent tasks
better than vanilla SkDebugf.

Example:
    ninja -C out ok; and out/ok gm 8888 filter:search=fontmgr_bounds trace -j -1

Change-Id: Ia3cdad930ce65e6fd12fa74f3fb00894e35138d3
Reviewed-on: https://skia-review.googlesource.com/26350
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed