Fix a stray cpplint warning

This somehow snuck in, preventing modifying Device.cpp without fixing
this warning. This needs to be done in a separate commit so the other
one can be cleanly reverted if needed.

Bug: None
Change-Id: I6755c889487ab63a727a8160dbbeda0cb2c129d6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/168044
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
diff --git a/src/dawn/native/Device.cpp b/src/dawn/native/Device.cpp
index 9ed8590..a23503b 100644
--- a/src/dawn/native/Device.cpp
+++ b/src/dawn/native/Device.cpp
@@ -30,6 +30,7 @@
 #include <algorithm>
 #include <array>
 #include <mutex>
+#include <unordered_set>
 #include <utility>
 
 #include "dawn/common/Log.h"