Fix unused variable warnings on ChromeOS

Fixes the build on ChromeOS where the #ifdefs there leave some
variables unused.

Bug: none
Change-Id: I7804fd15112e49a6bda1dadcb4137804409d1642
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/30463
Commit-Queue: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/vulkan/SwapChainVk.cpp

@@ -85,6 +85,11 @@ namespace dawn_native { namespace vulkan {
             const VulkanFunctions& fn = backend->GetFunctions();
             VkInstance instance = backend->GetVkInstance();
 
+            // May not be used in the platform-specific switches below.
+            DAWN_UNUSED(info);
+            DAWN_UNUSED(fn);
+            DAWN_UNUSED(instance);
+
             switch (surface->GetType()) {
 #if defined(DAWN_ENABLE_BACKEND_METAL)
                 case Surface::Type::MetalLayer: