[Regression] Fix Draggable is float:left and container is overflow:hidden

The change from https://chromiumcodereview.appspot.com/23537039 is
reverted (the cause for the regression). This patch also fixes the original
issue 23537039 tried to address. If the target element was a descendant of an 
inline-block the PaintInfo::shouldPaintWithinRoot did an early return from the 
paint call on inlineFlowBox. We let the paint call go through by clearing the 
paintRoot, on the basis that the inlineFlowBoxes will be intersecting the paint 
rect.

BUG=341089, 101204

Review URL: https://codereview.chromium.org/278293002

git-svn-id: svn://svn.chromium.org/blink/trunk@176613 bbb929c8-8fbe-4397-9dbb-9b2b20218538