This improves object binding performance. We no longer need to check
two containers when manipulating the bound subjects/observers.

Bug: angleproject:2763
Change-Id: I0ccb2e76be47bd8a28fba5a0c3eff857aa166bb7
Reviewed-on: https://chromium-review.googlesource.com/1227795
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>