Drag'n drop Autoscroll does not work properly on inner frames

RenderBox::calculateAutoscrollDirection does not properly translate
inner frames' coordinates in order to determine its auto-scrollability.
By coincidence, if the inner frame box it placed near to page's 0, 0 position
(upper left corner), it might work.

Patch fixes it by changing ::calculateAutoscrollDirection algorithm to operate
with window coordinates, taking inner frames offset and scroll position into account.
The behavior of auto-scrollable non-frame overflow boxes, including divs, still works
as it is used to.

Test: fast/events/drag-and-drop-autoscroll-inner-frame.html

BUG=chromium:286325

Review URL: https://chromiumcodereview.appspot.com/24074002

git-svn-id: svn://svn.chromium.org/blink/trunk@157535 bbb929c8-8fbe-4397-9dbb-9b2b20218538