This patch adds an additional check in the initialization of device
on D3D12 backend to ensure that 'use_dxc' is enabled only when Dawn
can load DXC DLLs (dxil.dll and dxcompiler.dll).
BUG=dawn:402
Change-Id: I85bc8698cf54b612244efc3673241f60a9cee209
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/33060
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>