Use Observer pattern for Buffers in front-end. This will allow us to react to state change notifications for validation caching. It also cleans up some of the logic in the D3D11 State Manager. Bug: angleproject:2747 Change-Id: I85ed6404206c2b9bf504d552cf5751be56e62146 Reviewed-on: https://chromium-review.googlesource.com/1172086 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>