Move Buffer Subject/Observer to front end. 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>