dawn: fix 'control reaches end of non-void function' warning

Change-Id: I8820df29d84df9151f7992677ca7e926d14ecb08
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/107240
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/dawn/native/vulkan/external_memory/MemoryService.cpp b/src/dawn/native/vulkan/external_memory/MemoryService.cpp
index 0db58f4..02361e2 100644
--- a/src/dawn/native/vulkan/external_memory/MemoryService.cpp
+++ b/src/dawn/native/vulkan/external_memory/MemoryService.cpp
@@ -52,6 +52,7 @@
             // for prefersDA.
             return dedicatedRequirements.prefersDedicatedAllocation;
     }
+    DAWN_UNREACHABLE();
 }
 
 }  // namespace dawn::native::vulkan::external_memory