D3D11: Use angle::Result error pattern. 2/3 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>