Coordinates-based snap offsets don't update correctly when container is scrolled

Coordinates-based snap offsets don't update correctly when container is scrolled
https://bugs.webkit.org/show_bug.cgi?id=147215

Reviewed by Brent Fulgham.

Source/WebCore:

Fixes the way we append the snap offsets of child elements with coordinates. We
now consider the scroll offset of the parent scroll snapping container, so snap
offset recomputations don't fail on scroll snapping containers.

Test: css3/scroll-snap/scroll-snap-coordinate-overflow-resize.html

* page/scrolling/AxisScrollSnapOffsets.cpp:
(WebCore::appendChildSnapOffsets): Fixed to consider the scroll offset of the
    parent container.

LayoutTests:

Tests that snap offsets update properly on scrolled containers.

* css3/scroll-snap/scroll-snap-coordinate-overflow-resize-expected.txt: Added.
* css3/scroll-snap/scroll-snap-coordinate-overflow-resize.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@187210 268f45cc-cd09-0410-ab3c-d52691b4dbfc