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>