Container objects in this case are the Vertex Array and Read and Draw
Framebuffers. Instead of using a mutable dirty bit state with a const
function to notify of dirty bits we can use the Observer/Subject
pattern. This more cleanly allows us to do cache updates.

Bug: angleproject:1391
Change-Id: I88f863894ec3efa00322038f323a84850166107d
Reviewed-on: https://chromium-review.googlesource.com/1153399Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>