This CL introduces d3d::Context. This new class can wrap a DisplayD3D
and a Context9/11 so we can use them through the same code paths. It
also refactors the D3D11 resource allocation to always have access to
a d3d::Context.

Bug: angleproject:2738
Change-Id: I8161acdffee7a868cc26f2d49715413bcca7cc2b
Reviewed-on: https://chromium-review.googlesource.com/1151451
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>