[macOS] Eliminate Vulkan hack for macOS < 10.14

[cbracken]

Eliminates an undef of VK_USE_PLATFORM_METAL_EXT that works around some unguarded @available checks for macOS 10.13. Our minimum macOS SDK is now macOS 10.14 so we can safely assume Metal support since it's a requirement for macOS 10.14.

No test changes since this should only affect the graphics backend used in tests; the tests themselves should be the same regardless of which backend is in use.

Issue: flutter/flutter#114445

Pre-launch Checklist

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• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[auto-submit]

auto label is removed for flutter/engine, pr: 37498, due to - The status or check suite Mac Host Engine has failed. Please fix the issues identified (or deflake) before re-applying this label.

[cbracken]

Seeing a segfault in the impeller tests Metal shader compile, it looks like:

[----------] 6 tests from Play/RuntimeStageTest
[ RUN      ] Play/RuntimeStageTest.CanRegisterStage/Vulkan
../../flutter/impeller/playground/playground_test.cc:17: Skipped
Playground doesn't support this backend type.
[  SKIPPED ] Play/RuntimeStageTest.CanRegisterStage/Vulkan (0 ms)
[ RUN      ] Play/RuntimeStageTest.CanCreatePipelineFromRuntimeStage/Metal
[ERROR:flutter/fml/backtrace.cc(108)] Caught signal SIGSEGV during program execution.
Frame 0: 0x7ff8110cd177 newRenderPipeline()
Frame 1: 0x7ff811159b7c __233-[MTLCompiler createVertexStageAndLinkPipelineWithFragment:fragmentVariant:vertexFunction:serializedVertexDescriptor:descriptor:destinationArchive:options:reflection:compileStatistics:fragmentCompileTimeData:error:completionHandler:]_block_invoke.1436
Frame 2: 0x7ff80854f0cc _dispatch_call_block_and_release
Frame 3: 0x7ff808550317 _dispatch_client_callout
Frame 4: 0x7ff808556317 _dispatch_lane_serial_drain
Frame 5: 0x7ff808556dfd _dispatch_lane_invoke
Frame 6: 0x7ff808560eee _dispatch_workloop_worker_thread
Frame 7: 0x7ff808703fd0 _pthread_wqthread
Frame 8: 0x7ff808702f57 start_wqthread

...

Failed Command:

/Volumes/Work/s/w/ir/cache/builder/src/out/host_profile/impeller_unittests --gtest_repeat=2 --gtest_shuffle

Exit Code: -11

The failure appears to not occur during the host_debug run but rather on the host_profile build. We do 2 shuffled runs. Random seed in case this is ordering specific:

Note: Randomizing tests' orders with a seed of 32951 .

[chinmaygarde]

@cbracken Can you rebase and try again please? I am actively looking into flutter/flutter#114872 ATM. The failure you are running into is not related to your patch.

[cbracken]

Rebased and re-pushed.