Don't try to load libvulkan on Mac

Only SwiftShader would be available on Mac.

Bug: chromium:1297154
Change-Id: I943a39dfdde4215ba0ea880bf08294f37f08ee42
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/80720
Reviewed-by: Shrek Shao <shrekshao@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn/native/vulkan/BackendVk.cpp

@@ -407,6 +407,12 @@ namespace dawn::native::vulkan {
 
         InstanceBase* instance = GetInstance();
         for (ICD icd : kICDs) {
+#if defined(DAWN_PLATFORM_MACOS)
+            // On Mac, we don't expect non-Swiftshader Vulkan to be available.
+            if (icd == ICD::None) {
+                continue;
+            }
+#endif  // defined(DAWN_PLATFORM_MACOS)
             if (options->forceSwiftShader && icd != ICD::SwiftShader) {
                 continue;
             }