Skia/Metal: Add base::Feature for Metal

The feature controls creation of a viz::MetalContextProvider, then
propagates to the SharedContextState.

Bug: 952063
Change-Id: I38d221c18aea003a1e3ff5f9c390d713f349ef09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582452
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654319}
10 files changed