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Add viz::MetalContextProvider, and add Metal to the Skia build by
default. Do not wire up any of this yet.

Bug: 952063
Change-Id: I897d052bdbf046114c1ddc0527c1d3cafc8777c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577296Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652899}