Switch one more debug message UNREACHABLE to ASSERT(false)

Bug: chromium:1375131
Change-Id: I9150ef25ea3278d97f785f6ffbcc8d45ab79fd6a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/110703
Auto-Submit: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn/native/vulkan/DeviceVk.cpp

diff --git a/src/dawn/native/vulkan/DeviceVk.cpp b/src/dawn/native/vulkan/DeviceVk.cpp
index ffb058d..fed1cb9 100644
--- a/src/dawn/native/vulkan/DeviceVk.cpp
+++ b/src/dawn/native/vulkan/DeviceVk.cpp
@@ -1048,7 +1048,7 @@
     }
 
     // Crash in debug
-    UNREACHABLE();
+    ASSERT(false);
 }
 
 MaybeError Device::WaitForIdleForDestruction() {