D3D: Refactor VertexBuffer::getSpaceRequired. By making this a virtual call to BufferFactoryD3D (aka RendererD3D), we can also stop having side-effects in the BufferD3D class of creating a static buffer storage when we only want to know the space required for some vertex elements. This refactoring will aid implementation of VertexArray11 dirty bits. BUG=angleproject:1327 Change-Id: I0e34c6e9f5da35edebc179d578ad9392dc0166db Reviewed-on: https://chromium-review.googlesource.com/329741 Reviewed-by: Zhenyao Mo <zmo@chromium.org>