[LFC] Construct the Display::Box objects on demand.

[LFC] Construct the Display::Box objects on demand.
https://bugs.webkit.org/show_bug.cgi?id=189320

Reviewed by Antti Koivisto.

* layout/FormattingContext.cpp:
(WebCore::Layout::FormattingContext::layoutOutOfFlowDescendants const):
* layout/LayoutContext.cpp:
(WebCore::Layout::LayoutContext::initializeRoot):
(WebCore::Layout::LayoutContext::displayBoxForLayoutBox const):
(WebCore::Layout::LayoutContext::createDisplayBox): Deleted.
* layout/LayoutContext.h:
(WebCore::Layout::LayoutContext::displayBoxForLayoutBox const): Deleted.
* layout/Verification.cpp:
(WebCore::Layout::LayoutContext::verifyAndOutputMismatchingLayoutTree const):
* layout/blockformatting/BlockFormattingContext.cpp:
(WebCore::Layout::BlockFormattingContext::layout const):
* layout/displaytree/DisplayBox.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@235711 268f45cc-cd09-0410-ab3c-d52691b4dbfc