D3D11: Refactor Buffer classes.

- GPUOnlyBuffer is renamed to GPUUsableBuffer. This will allow it
to be extended by a subclass in future which supports mapping
a draw buffer (with uniform, index, storage usages, etc).
- MapInternal now has additional MapMode parameter.
- Clear function used for Occlusion Query should be a special
function. It will be named PredicatedClear. To indicate that the
clearing will be affected by a predicate, unlike mapping based Clear
which won't be affected by it.
- GetD3D11ConstantBuffer() and EnsureConstantBufferIsUpdated() are
combined. They are currently always called together, there is no point
having two separate methods.
- Similarly for CreateD3D11UnorderedAccessView1() and MarkMutated().

Bug: 345471009
Change-Id: Ifa237e6bacb591a1ee8e9b9f76c2e5d809899efc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/194800
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Quyen Le <lehoangquyen@chromium.org>