Add portable allocator to raster allocator test.

Still use GDI and CG when available, but use a Skia backed native raster
allocator on other platforms. This allows for exercising this code on
all platforms as well as ensuring that Skia itself can back this
interface. This also means this gm should always draw something instead
of often just being the white background.

Change-Id: Ie607f7082dc40454fc60ea4b4fe94a1db2d7502f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/234662
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed