This makes BufferImpl into an Observer Subject. It also refactors
the Vertex Array updates for the D3D11 backend use more of a dirty
bit coding style.

This change makes it so Buffer contents changes trigger front-end
dirty bits from the back-end, which may be undesirable.

Bug: angleproject:2389
Change-Id: Iac8ce1171284a86851c18cd1373ddf24fcefe40b
Reviewed-on: https://chromium-review.googlesource.com/979812
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>