Make gl::Buffer an angle::Subject. This will allow us to share the BindingPointer and ObserverBinding. It also cleans up the code a little bit. Bug: angleproject:2891 Change-Id: Ib8e2c34ccd379dc5b5648bac324fa4ff22a5a1a0 Reviewed-on: https://chromium-review.googlesource.com/c/1270216 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>