Revert "Skia/Metal raster: Add viz::MetalContextProvider"

This reverts commit 45f012f356246f0bc8962cbbc5c3c16263bdab76.

Reason for revert: Broke mac compile

Original change's description:
> Skia/Metal raster: Add viz::MetalContextProvider
> 
> 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/+/1577296
> Reviewed-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}

TBR=ccameron@chromium.org,brianosman@google.com,ericrk@chromium.org

Change-Id: Ic0a4f5201df55ac7ef0b8a191b572a9bbeee6dc0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 952063
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577783
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652909}