Use the Observer pattern to get notified when the BufferVk we've copied our data
from is changing. We will only recopy the data if anything has changed, but otherwise
we'll keep drawing with the same index buffer we've created previously.

Bug: angleproject:2335

Change-Id: Ib65677b4d5ec90c46a5e3b975fffd1fddeed59e7
Reviewed-on: https://chromium-review.googlesource.com/948622
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>