[iOS] Ignore pixel alignments when updating the MainContentUIState.

UIScrollView does not send a |-didEndDecelerating| to its delegate after
pixel alignments, so the MainContentUIState should not be considered
decelerating if the target contentOffset is less than a pixel away from
the current value.

This will correctly update the MainContentUIState's |scrolling|
property to NO, allowing the FullscreenScrollEndAnimator to start.

Bug: 800757
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I4407bf34c4a6c5cb485bba01528c34cf9e86c156
Reviewed-on: https://chromium-review.googlesource.com/922888
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#537382}(cherry picked from commit 24ec7cd715920382c1ab368cfc98efafc9530dd1)
Reviewed-on: https://chromium-review.googlesource.com/934942
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#572}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
4 files changed