[Impeller] Metal golden tests are unconditionally skipped on macOS when wide gamut is unsupported #191209

In golden_playground_test_mac.cc, GoldenPlaygroundTest::SetUp() unconditionally checks !DoesSupportWideGamutTests() for Metal and MetalSDF backends:

case PlaygroundBackend::kMetalSDF:
  switches.flags.use_sdfs = true;
  [[fallthrough]];
case PlaygroundBackend::kMetal:
  if (!DoesSupportWideGamutTests()) {
    GTEST_SKIP()
        << "This metal device doesn't support wide gamut golden tests.";
  }

Since DoesSupportWideGamutTests() returns false on non-ARM64 setups (such as Mac_x64 CI runners and Intel Macs), this causes all Metal and MetalSDF golden tests to be skipped on machines that don't support wide gamut.

However, only 5 of the ~460 golden tests actually require wide gamut rendering (WideGamut_*). Standard sRGB Metal golden tests do not require wide gamut and should execute normally.

This check was introduced in flutter/engine#52142. The other backends in the same file (kVulkan and kOpenGLES) already guard their wide gamut checks with if (switches.enable_wide_gamut). So I propose we do the same for Metal