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The embedded metallib uses MTLLanguageVersion2_1, available only
macOS 10.14+.

Bug: 1046671
Change-Id: Iac72569f4d9ad25ced7c0284dbbfd790ab3ecd4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028266Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736512}