[dawn] Minor fix for sizeof on bitfield

Looks like when dawn assert are disabled we fall down into a
macro for DAWN_ASSERT that uses sizeof(conditional).

This for logical reasons is not allowed.

I not could find an easy fix to Assert.h so lets fix this for now
this way.

Change-Id: Ibd901bcb1e44b7f041455c9776bf5107f858ae20
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/276474
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Peter McNeeley <petermcneeley@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
diff --git a/src/dawn/native/vulkan/BufferVk.cpp b/src/dawn/native/vulkan/BufferVk.cpp
index f53175f..e39753d 100644
--- a/src/dawn/native/vulkan/BufferVk.cpp
+++ b/src/dawn/native/vulkan/BufferVk.cpp
@@ -682,7 +682,7 @@
     Device* device = ToBackend(GetDevice());
     const bool isMappable = GetInternalUsage() & kMappableBufferUsages;
 
-    DAWN_ASSERT(mHostVisible);
+    DAWN_ASSERT(!!mHostVisible);
     DAWN_ASSERT(GetLastUsageSerial() <= device->GetQueue()->GetCompletedCommandSerial());
 
     VkDeviceMemory deviceMemory = ToBackend(mMemoryAllocation.GetResourceHeap())->GetMemory();