[LFC] The static position for an out-of-flow box should include the previous sibling...

[LFC] The static position for an out-of-flow box should include the previous sibling's collapsed margin
https://bugs.webkit.org/show_bug.cgi?id=187169

Reviewed by Antti Koivisto.

When computing the static position of an absolutely positioned box, we need to look at the previous sibling's bottom margin.
If the previous sibling happens to collapse its bottom margin with the parent's bottom margin, we still need to account for it
and compute the static vertical position as if the bottom margin was not collapsed.

* layout/FormattingContext.cpp:
(WebCore::Layout::FormattingContext::computeFloatingHeightAndMargin const):
(WebCore::Layout::FormattingContext::computeOutOfFlowVerticalGeometry const):
* layout/FormattingContextGeometry.cpp:
(WebCore::Layout::staticVerticalPositionForOutOfFlowPositioned):
* layout/LayoutContext.cpp:
(WebCore::Layout::LayoutContext::initializeRoot):
* layout/Verification.cpp:
(WebCore::Layout::outputMismatchingBoxInformationIfNeeded):
* layout/blockformatting/BlockFormattingContext.cpp:
(WebCore::Layout::BlockFormattingContext::computeInFlowHeightAndMargin const):
* layout/displaytree/DisplayBox.cpp:
(WebCore::Display::Box::nonCollapsedMarginBox const):
* layout/displaytree/DisplayBox.h:
(WebCore::Display::Box::setHasValidVerticalNonCollapsedMargin):
(WebCore::Display::Box::setVerticalMargin):
(WebCore::Display::Box::setVerticalNonCollapsedMargin):
(WebCore::Display::Box::nonCollapsedMarginTop const):
(WebCore::Display::Box::nonCollapsedMarginBottom const):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@233350 268f45cc-cd09-0410-ab3c-d52691b4dbfc