Two tests are added to measure the overhead involved in the
Observer/Subject interface -
1. FramebufferAttachmentBenchmark - stresses repeated attach/detach
   of observers to subjects.
2. FramebufferAttachmentStateUpdateBenchmark - stresses state updates
   to subjects and their propagation to observers.

Bug: angleproject:5692
Change-Id: I6e573f3a948ef2b20c56091e549eb871051e0103
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2725740
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>