Add support/docs for profiling GPU process

I was trying to debug the linked regression and was unable to use
the existing cpu profiling support for the GPU process (where Skia
runs) so I added it.

I also added some docs as I didn't find any existing docs related
to profiling in my search (happy to be shown where those exist).

Bug: chromium:504647815
Change-Id: I6d673f4c9780b28b71f76741b359758a4be3a3f9
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/7830384
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Kaylee Lubick <kjlubick@chromium.org>