Add skia_enable_discrete_gpu argument to GN

This forces all of our testing tools to run with the discrete GPU in
laptop systems that have that option.

BUG=skia:

Change-Id: Ibd7629d6de5f063cdf219b3c7469210af5085d90
Reviewed-on: https://skia-review.googlesource.com/6474
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed