Fix scrollbar thumb drag cancellation in RTL mode

Scrollbar stores original scroll position in non-negative scroll coordinates.
When sending those coordinates to ScrollableArea, they should be offsetted
with minimumScrollPosition() first.

R=jchaffraix@chromium.org
BUG=377131

Review URL: https://codereview.chromium.org/302603009

git-svn-id: svn://svn.chromium.org/blink/trunk@175100 bbb929c8-8fbe-4397-9dbb-9b2b20218538