Simplify gn_args in gn_to_bp.py

Bug: b/118742766

This does not change Android.bp or (any) SkUserConfig.h. But it
consolidates the arguments for the different platforms into one method,
which makes it clear what they share and where they differ. It also no
longer sets skia_enable_tools for the non-Android build, where it
doesn't need to be set to true (this makes no difference in the final
output nor the time GN takes to generate JSON).

Change-Id: I22af345aaf3878227d4f42afc2861e49ec2691ec
Reviewed-on: https://skia-review.googlesource.com/c/173238
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
1 file changed