refactor RenderFlexibleBox to return preferred sizes of all children

refactor RenderFlexibleBox to return preferred sizes of all children
https://bugs.webkit.org/show_bug.cgi?id=78169

Reviewed by Ojan Vafai.

This is in preparation for multi-line flexbox. We need the preferred
size of each child so we can compute where the multi-line breaks happen.

No new tests, just refactoring.

* rendering/RenderFlexibleBox.cpp:
(WebCore::RenderFlexibleBox::preferredMainAxisContentExtentForChild): Renamed from preferredMainAxisContentExtentForFlexItem because
other methods are ForChild rather than ForFlexItem.
(WebCore::RenderFlexibleBox::layoutFlexItems):
(WebCore::RenderFlexibleBox::computeMainAxisPreferredSizes):
(WebCore::RenderFlexibleBox::runFreeSpaceAllocationAlgorithm):
* rendering/RenderFlexibleBox.h:
(RenderFlexibleBox):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107179 268f45cc-cd09-0410-ab3c-d52691b4dbfc