Remove TreeOrderIterator and iterate over the child boxes directly.

Remove TreeOrderIterator and iterate over the child boxes directly.
https://bugs.webkit.org/show_bug.cgi?id=78294

Reviewed by Tony Chang.

No new tests. This is purely a refactor. No change in behavior.

* rendering/RenderFlexibleBox.cpp:
(WebCore::RenderFlexibleBox::layoutFlexItems):
(WebCore::RenderFlexibleBox::computeMainAxisPreferredSizes):
* rendering/RenderFlexibleBox.h:
(RenderFlexibleBox):
(FlexOrderHashTraits):
(WebCore::RenderFlexibleBox::FlexOrderHashTraits::emptyValue):
(WebCore::RenderFlexibleBox::FlexOrderHashTraits::constructDeletedValue):
(WebCore::RenderFlexibleBox::FlexOrderHashTraits::isDeletedValue):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@107317 268f45cc-cd09-0410-ab3c-d52691b4dbfc