D3D12 Immediately release pending commands

We want to release pending commands in Device::WaitForIdleForDestruction()
so that when we call TickImpl(), we can reset the command allocators.

Bug: dawn:269
Change-Id: Ibd8fdd685a4e9ed7cce8176ba097bc67687bcd97
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/18101
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/d3d12/DeviceD3D12.cpp

@@ -416,6 +416,9 @@ namespace dawn_native { namespace d3d12 {
     }
 
     MaybeError Device::WaitForIdleForDestruction() {
+        // Immediately forget about all pending commands
+        mPendingCommands.Release();
+
         DAWN_TRY(NextSerial());
         // Wait for all in-flight commands to finish executing
         DAWN_TRY(WaitForSerial(mLastSubmittedSerial));