Add pop_back() / back() method for FixedVector Good for implementing stacks of fixed size (in the pop operation). + fix an issue where push_back() only allows adding up to max_size() - 1 elements. BUG=angleproject:2306 Change-Id: I99b3c8416055f069fa563b684c102fd6aec116bc Reviewed-on: https://chromium-review.googlesource.com/1000031 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Lingfeng Yang <lfy@google.com>