GitLab

Finishes up a few left behind works from the first cl.

1. Handles window items that are still alive but only for animation
purposes by ignoring them in PositionWindows. Previously had some
indexing errors and the windows would bounce around if swipe was used
too quickly.

2. Handles one more case where nudge should not occur - when an item
from the previous row drops down to the nudging row, which looks funny.

3. Use the nudging function described in the bug instead of just linear
interpolation.

Test: added tests
Bug: 843273
Change-Id: I9d2088083a71779fc78f72309b5674f35bc162c7
Reviewed-on: https://chromium-review.googlesource.com/1100389
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567462}