[dawn][emscripten] Add [[maybe_unused]] for debug assert.

- In opt builds, the variable was unused and caused compilation
  warning -> error.

No-Try: true
Change-Id: I684859847de874cfe53bbb5e2c1d4af9e5b085d8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/242394
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Auto-Submit: Loko Kung <lokokung@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>

third_party/emdawnwebgpu/pkg/webgpu/src/webgpu.cpp

diff --git a/third_party/emdawnwebgpu/pkg/webgpu/src/webgpu.cpp b/third_party/emdawnwebgpu/pkg/webgpu/src/webgpu.cpp
index d1f9d62..5b8a489 100644
--- a/third_party/emdawnwebgpu/pkg/webgpu/src/webgpu.cpp
+++ b/third_party/emdawnwebgpu/pkg/webgpu/src/webgpu.cpp
@@ -1366,7 +1366,7 @@
         WGPUDeviceLostReason_FailedCreation, "Device creation failed.");
 
     // Free the device now that there should be no pointers to it.
-    bool deviceFreed = device->Release();
+    [[maybe_unused]] bool deviceFreed = device->Release();
     assert(deviceFreed);
   }
 }