https://bugs.webkit.org/show_bug.cgi?id=118176
<rdar://problem/14301271>

Reviewed by Anders Carlsson.

Test: compositing/geometry/fixed-position-flipped-writing-mode.html

WebCore makes use of constrainScrollPositionForOverhang not only for
constraining fixed- and sticky-positioned elements to the viewport, but
also for clamping the tile cache's visible rect.

Therefore, constrainScrollPositionForOverhang needs to correctly take
the scrollOrigin into account. The easiest way I saw to do this was to
reimplement the function in terms of a pair of rect intersections
between a virtual scrollable "viewport" and the document (with a bit
of complication from headers and footers).

The first intersection is performed, then if the viewport doesn't fit,
it is pushed down and to the right, from the origin. Next, we intersect
again, this time pushing the rect up by the amount it overflowed the document
rect from the bottom right. This performs effectively the same constraint
as previously, but handles the scrollOrigin correctly and is also somewhat
easier to read and understand (with pictures).

* page/scrolling/mac/ScrollingTreeScrollingNodeMac.mm:
(WebCore::ScrollingTreeScrollingNodeMac::setScrollLayerPosition):
Subtract the scrollOrigin out of the offset passed to scrollOffsetForFixedPosition,
it's expecting an offset without the origin included.

* platform/ScrollableArea.cpp:
(WebCore::ScrollableArea::constrainScrollPositionForOverhang):
Reimplement the function as described above.

Add a test that ensures that fixed position works with a scrollable document
and direction: rtl, which is one case where scrollOrigin is set.

Disable the test on Mac WebKit1 because of https://bugs.webkit.org/show_bug.cgi?id=118269.

* compositing/geometry/fixed-position-flipped-writing-mode-expected.txt: Added.
* compositing/geometry/fixed-position-flipped-writing-mode.html: Added.
* platform/mac/compositing/geometry/fixed-position-flipped-writing-mode-expected.png: Added.
* platform/mac/TestExpectations:
* platform/mac-wk2/TestExpectations:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@152303 268f45cc-cd09-0410-ab3c-d52691b4dbfc