Do not use flat interpolation with ANGLE's metal backend

Using flat interpolation is slow with ANGLE's metal backend,
so not using it.

Bug: chromium:1340620
Change-Id: I91ff04a975ca9778690c6372d240028f24b7c2a0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/554976
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed