Unify TickImpl called during Device shutdown.

Bug: dawn:400
Change-Id: I26ad1a739d123ebc81a2e7d309c8a42f517be06c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/22662
Commit-Queue: Natasha Lee <natlee@microsoft.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/null/DeviceNull.cpp

@@ -185,6 +185,7 @@ namespace dawn_native { namespace null {
     }
 
     MaybeError Device::WaitForIdleForDestruction() {
+        mPendingOperations.clear();
         return {};
     }