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/329741Reviewed-by: Zhenyao Mo <zmo@chromium.org>
