Buffer: Notify contents changed in a separate observer list. Buffer contents changed will only notify VertexArray. Also this list is separated from the other observers because in the future most of the time we won't need to notify the VertexArrays when buffer contents changed, to avoid iterating over a large list of VertexArray observers on simple SubData calls. Bug: angleproject:6371 Change-Id: I6142a5757868caf33dc6594a1975f817f80553e8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3182701 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>