Reset Internal D3D12 Device After Test Shutdown

Adds dawn_native::ResetDeviceInternal, which allows us to destroy and
create a new ID3D12Device. The device should be reset after every test
when GPU-based validation is enabled in order to prevent GBV objects
from using a significant amount of memory over time.

Bug: dawn:623
Change-Id: I654d093d993ab0198c6c240bd0f3f843d2762680
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/41340
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Commit-Queue: Brandon Jones <brandon1.jones@intel.com>

src/include/dawn_native/DawnNative.h

@@ -116,6 +116,9 @@ namespace dawn_native {
         // On an error, nullptr is returned.
         WGPUDevice CreateDevice(const DeviceDescriptor* deviceDescriptor = nullptr);
 
+        // Reset the backend device object for testing purposes.
+        void ResetInternalDeviceForTesting();
+
       private:
         AdapterBase* mImpl = nullptr;
     };