[lFC][Floating] Add estimated margin top computation.

[lFC][Floating] Add estimated margin top computation.
https://bugs.webkit.org/show_bug.cgi?id=188619

Reviewed by Antti Koivisto.

In order to figure out whether a box should avoid a float, we need to know the final positions of both (ignore relative positioning for now).
In block formatting context the final position for a normal flow box includes
1. the static position and
2. the corresponding (non)collapsed margins.
Now the vertical margins are computed when all the descendants are finalized, because the margin values might be depending on the height of the box
(and the height might be based on the content).
So when we get to the point where we intersect the box with the float to decide if the box needs to move, we don't yet have the final vertical position.

The idea here is that as long as we don't cross the block formatting context boundary, we should be able to pre-compute the final top margin.
(if this holds true for all the cases, the estimated prefix could be removed and just use marginTop() instead.)

Covered by existing block-only tests.

* layout/blockformatting/BlockFormattingContext.cpp:
(WebCore::Layout::BlockFormattingContext::layout const):
(WebCore::Layout::BlockFormattingContext::computeEstimatedMarginTop const):
(WebCore::Layout::BlockFormattingContext::computeEstimatedMarginTopForAncestors const):
(WebCore::Layout::BlockFormattingContext::computeFloatingPosition const):
(WebCore::Layout::BlockFormattingContext::computeVerticalPositionForFloatClear const):
(WebCore::Layout::BlockFormattingContext::computeInFlowHeightAndMargin const):
* layout/blockformatting/BlockFormattingContext.h:
* layout/blockformatting/BlockMarginCollapse.cpp:
(WebCore::Layout::BlockFormattingContext::MarginCollapse::estimatedMarginTop):
* layout/displaytree/DisplayBox.cpp:
(WebCore::Display::Box::Box):
* layout/displaytree/DisplayBox.h:
(WebCore::Display::Box::setHasValidTop):
(WebCore::Display::Box::setHasValidLeft):
(WebCore::Display::Box::top const):
(WebCore::Display::Box::left const):
(WebCore::Display::Box::topLeft const):
(WebCore::Display::Box::setTopLeft):
(WebCore::Display::Box::setTop):
(WebCore::Display::Box::setLeft):
(WebCore::Display::Box::setVerticalMargin):
(WebCore::Display::Box::setEstimatedMarginTop):
(WebCore::Display::Box::estimatedMarginTop const):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@234927 268f45cc-cd09-0410-ab3c-d52691b4dbfc