Handle DeviceLost error

Handle DeviceLostCallback once DeviceLost error occurs.
Disallow any other commands or actions on device to happen after device
has been lost.

Bug: dawn:68
Change-Id: Icbbbadf278cae5e6213050d00439118789c863dc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/12801
Commit-Queue: Natasha Lee <natlee@microsoft.com>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn_native/null/DeviceNull.cpp

@@ -166,6 +166,8 @@ namespace dawn_native { namespace null {
     }
 
     void Device::Destroy() {
+        ASSERT(mLossStatus != LossStatus::AlreadyLost);
+
         mDynamicUploader = nullptr;
 
         mPendingOperations.clear();