Null: Properly fake commands being completed on WaitForIdle

Bug: chromium:1068465
Change-Id: I7c01b1c4fed2424a53d30b7e53dabf4d74ed4a73
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/19288
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/null/DeviceNull.cpp

@@ -188,6 +188,8 @@ namespace dawn_native { namespace null {
     }
 
     MaybeError Device::WaitForIdleForDestruction() {
+        // Fake all commands being completed
+        mCompletedSerial = mLastSubmittedSerial;
         return {};
     }