Fix fling behavior for banners

If the user does a scroll or fling that tries to interrupt an animation,
the action gets dropped.  Instead, cancel the animation when it's reasonable
to do so and begin reacting to the user.

Also, tweak how the view determines whether it should hide after being flung
to try and get closer behavior to the omnibox.

BUG=341556
R=dtrainor@chromium.org

Review URL: https://codereview.chromium.org/177243019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254895 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254895