message: Fix availability check for MTLCopyAllDevices
  This caused failure on macOS 10.11 because Dawn decided Metal is
  supported but would go ahead and reach an UNREACHABLE() block.
diff: @@ -220,7 +220,7 @@ namespace dawn_native { namespace metal {
  std::vector<std::unique_ptr<AdapterBase>> Backend::DiscoverDefaultAdapters() {
  std::vector<std::unique_ptr<AdapterBase>> adapters;
  - if (@available(macOS 10.12, *)) {
  + if (@available(macOS 10.11, *)) {
  NSArray<id<MTLDevice>>* devices = MTLCopyAllDevices();
  for (id<MTLDevice> device in devices) {
  