[iOS] Don't use pointers for FullscreenControllerImpl ivars.

This was originally done to avoid importing the headers from the
fullscreen_controller.h since these classes are not in the public
fullscreen target.  However, since FullscreenController was converted
to a public interface and FullscreenControllerImpl moved to the
private target, this isn't necessary anymore.

This CL also updates FullscreenWebStateListObserver to manage the
WebStateList, as it's not used by FullscreenControllerImpl.

Bug: none
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ib97bd0a3b31d6a54fab7e3588aaed53b81b714d4
Reviewed-on: https://chromium-review.googlesource.com/1219994
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590750}