Enable direct-to-metal backend by default We are switching over to Apple's direct-to-metal backend instead of generating SPIRV in the metal backend. This CL enables the direct-to-metal generation by default, but the SPIRV backend is still accessible by overriding the feature directMetalGeneration. This CL comes with a change in test expectations to catch new failures and clean up newly passing tests. Bug: angleproject:6080 Change-Id: I4b10ad93c641b88857079a08fb45d3dc575d71f1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3175664 Reviewed-by: Gregg Tavares <gman@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>