Use fragmented bounding box for hit-test/paint clipping.

When calculating the bounding box used for hit testing and paint clipping,
use the visual bounding box from the union of the fragments instead of the
flow thread coordinates.

The physicalBoundingBox method returned a bounding box in flow thread
coordinates and not visual coordinates. Rename it to boundingBox and
introduce a fragmentedBoundingBox to find the visual one.

Fixed a bug negation bug in previously unused convertToLayerCoords that
takes a LayoutRect.

This fixes the regressions introduced by 18e0c42d.

R=mstensho@opera.com
BUG=359877

Review URL: https://codereview.chromium.org/612323011

git-svn-id: svn://svn.chromium.org/blink/trunk@183251 bbb929c8-8fbe-4397-9dbb-9b2b20218538