Change UNREACHABLE to ASSERT(false) in OnDebugUtilsCallback

In cases that a debug message does happen, we shouldn't use
unreachable as it will make the compiler use a false assumption.
Instead, only ASSERT(false) which crashes in debug, and does
nothing in release builds.

Bug: chromium:1375131
Change-Id: I7733151c241ee875ac40969ce22f037351141e89
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/110600
Auto-Submit: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
diff --git a/src/dawn/native/vulkan/BackendVk.cpp b/src/dawn/native/vulkan/BackendVk.cpp
index 870824f..e2ef5c6 100644
--- a/src/dawn/native/vulkan/BackendVk.cpp
+++ b/src/dawn/native/vulkan/BackendVk.cpp
@@ -157,7 +157,7 @@
     // We get to this line if no device was associated with the message. Crash so that the failure
     // is loud and makes tests fail in Debug.
     dawn::ErrorLog() << pCallbackData->pMessage;
-    UNREACHABLE();
+    ASSERT(false);
 
     return VK_FALSE;
 }