GitLab

Design:
https://docs.google.com/document/d/1fZOg120fswUdaV7tfZh8cXIlcvAL1Gg70GVrsQVXl3g

This is the MVP of an animating layout. Feature supported include:
 - layout can animate its preferred size or fit the size given
 - preferred size (if applicable) and child view bounds automatically
   animate from one proposed layout to the next
 - No animation is performed if the host view's size is set externally.
 - No fading of views being removed or hidden is performed; a view being
   hidden or removed disappears right away, and a view being shown
   appears at the end of the animation (to prevent child views from
   overlapping).

Future work:
 - Selectively animate child views when the host view's size is set
   externally.
 - Fade child views in/out as they are added/removed or shown/hidden.
 - Maybe handle nested animating layouts.

Bug: 898632
Change-Id: I43873fabbce47ed247626613885a59fe1ca299e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742509Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687336}