Add the ability to customize skia's settings in gn

Fuchsia also builds Skia for host and wants to set various gn variables (e.g.,
skia_enable_flutter_defines) when is_linux is true. Rather than teach Skia
about all these permutations, this CL adds a skia_settings variable that can be
used by clients to customize Skia's configuration.

Change-Id: I6fd4798b0ce86615688ea19328236ae0f3a93c4d
Reviewed-on: https://skia-review.googlesource.com/59720
Commit-Queue: Adam Barth <abarth@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed