[iOS] Fix FullscreenModel::UpdateDisabledCounterForContentHeight().

This CL updates UpdateDisabledCounterForContentHeight()'s implementation
in two ways:
- Accounts for the case when the FullscreenProvider is disabled and the
  scroll view may have been resized to a shorter height despite the
  content being fully visible on screen.
- Updates to a <= check so that fullscreen is disabled when the content
  fits the scroll view exactly.

Bug: 914347
Change-Id: I5d2c6e3ce09d25552de0f1071785d29078162377
Reviewed-on: https://chromium-review.googlesource.com/c/1423092
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625360}
5 files changed