[iOS] Use UIScrollView scroll to top animation.

If you animate a UIScrollView's contentOffset while it's decelerating,
the CADisplayLink powering the deceleration scrolling is not stopped.
As a result, the deceleration will continue after the contentOffset
animation is completed.  This CL updates fullscreen's scroll to top
animation to allow the built-in UIScrollView scroll to top animation,
which cancels the deleration CADisplayLink.

Bug: 811294
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I2fbb572dca7963e760b3ec417eb3157d9e4a5baf
Reviewed-on: https://chromium-review.googlesource.com/922273
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#537114}(cherry picked from commit cb56b831ff407cb2396173944607b4ee7f6a2a80)
Reviewed-on: https://chromium-review.googlesource.com/935161
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#573}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
2 files changed