Will be useful for an optimization to the Buffer Subject/Observer
pattern.

Also cleans up an ASSERT.

Bug: angleproject:2389
Bug: chromium:829906
Change-Id: I2f8313ab531bca61947a51cc2396c04fb5d4bb1d
Reviewed-on: https://chromium-review.googlesource.com/1002883Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Lingfeng Yang <lfy@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>