d3d11: implement Adaptor, Backend, etc

This CL also moves some comman code in d3d12::Adapter and
d3d11::Backend to d3d::Adapter and d3d::Backend, so it can be
shared with d3d11 backend.

Bug: dawn:1705
Change-Id: Ica3bf33232d3380b7d4799c77bf9b43a8374a66e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/126220
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>

scripts/dawn_features.gni

@@ -54,6 +54,9 @@ declare_args() {
   # standalone to produce static libraries to use in their projects.
   dawn_complete_static_libs = false
 
+  # Enables the compilation of Dawn's D3D11 backend
+  dawn_enable_d3d11 = false
+
   # Enables the compilation of Dawn's D3D12 backend
   dawn_enable_d3d12 = is_win