message: Fix Apple SDK availablity check
diff: @@ -229,9 +229,10 @@ namespace dawn_native { namespace metal {
  std::vector<std::unique_ptr<AdapterBase>> Backend::DiscoverDefaultAdapters() {
  std::vector<std::unique_ptr<AdapterBase>> adapters;
  -
  - if (@available(macOS 10.11, *)) {
  + BOOL supportedVersion = NO;
  #if defined(DAWN_PLATFORM_MACOS)
  + if (@available(macOS 10.11, *)) {
  + supportedVersion = YES;
  NSArray<id<MTLDevice>>* devices = MTLCopyAllDevices();
  for (id<MTLDevice> device in devices) {
  @@ -239,14 +240,18 @@ namespace dawn_native { namespace metal {
  }
  [devices release];
  + }
  #endif
  - } else if (@available(iOS 8.0, *)) {
  +
  #if defined(DAWN_PLATFORM_IOS)
  + if (@available(iOS 8.0, *)) {
  + supportedVersion = YES;
  // iOS only has a single device so MTLCopyAllDevices doesn't exist there.
  adapters.push_back(
  std::make_unique<Adapter>(GetInstance(), MTLCreateSystemDefaultDevice()));
  + }
  #endif
  - } else {
  + if (!supportedVersion) {
  UNREACHABLE();
  }
  return adapters;
  