Windows should animate when they are about to get docked at screen edges.

Windows should animate when they are about to get docked at screen edges.
This CL also changes the logic of reparenting windows during a drag. Regular windows (not panels) are reparented to docked container during the drag (so they float above other windows) and are then reparented back to default container (if they are not docked) or stay in the docked container (if they stay or become docked at the end of the drag). Panels are reparented to panel container during a drag and then get reparented back to default container at the end of the drag (if they are not attached to shelf) or to docked container (if they are docked) or stay in panel container (if they are attached to shelf).
This change was necessary in this CL to properly show animation and docked area background while dragging.

BUG=264024

Review URL: https://chromiumcodereview.appspot.com/20180002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@217336 0039d316-1c4b-4281-b951-d872f2087c98