Fix mFenceEvent HANDLE leak

dawn_native::d3d12::Device::mFenceEvent is created in Initialize
but is never closed.

Fix is to close in the destructor if it has been previously created.

Bug: dawn:215
Change-Id: Ie9e7a395fe6fe791e2581295ed12f20070671afa
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/10480
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/d3d12/DeviceD3D12.cpp

@@ -120,6 +120,10 @@ namespace dawn_native { namespace d3d12 {
         // Call Tick() again to clear them before releasing the allocator.
         mResourceAllocator->Tick(mCompletedSerial);
 
+        if (mFenceEvent != nullptr) {
+            ::CloseHandle(mFenceEvent);
+        }
+
         ASSERT(mUsedComObjectRefs.Empty());
         ASSERT(mPendingCommands.commandList == nullptr);
     }

src/dawn_native/d3d12/DeviceD3D12.h

@@ -107,7 +107,7 @@ namespace dawn_native { namespace d3d12 {
         Serial mCompletedSerial = 0;
         Serial mLastSubmittedSerial = 0;
         ComPtr<ID3D12Fence> mFence;
-        HANDLE mFenceEvent;
+        HANDLE mFenceEvent = nullptr;
 
         ComPtr<ID3D12Device> mD3d12Device;  // Device is owned by adapter and will not be outlived.
         ComPtr<ID3D12CommandQueue> mCommandQueue;