[iOS] Don't disable the FullscreenModel mid-scroll for short content.

When the viewport grows as the toolbars are collapsed for fullscreen,
sometimes the scroll view height is greater than the content height
because the content renders asynchronously and hasn't been updated for
the new viewport.  Don't disable the model when this occurs during
scroll events.  If the content remains shorter than the scroll view at
the beginning of the next scroll, the model should be disabled then.

Bug: 960895
Change-Id: I6512a39fc0c82e12934bce01aed56a02578ab0be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600343
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657883}
2 files changed