Fix smooth scroll overshooting when mouse held down in scrollbar track.
Scrollbar::autoscrollPressedPart now checks if the mouse is under the thumb's
eventual (target) position, rather than its current position.
BUG=
578554
Committed:
https://crrev.com/ef53ba38118cb84ed66bb9d9335e8b3a550865d6
Cr-Commit-Position: refs/heads/master@{#370285}