Make RenderBlock::paintChildren virtual so sub classes can change the paint order

Make RenderBlock::paintChildren virtual so sub classes can change the paint order
https://bugs.webkit.org/show_bug.cgi?id=92042

Reviewed by Ojan Vafai.

The flexbox spec says that the order property should change the paint order of flex children.
To make that possible, make paintChildren virtual so RenderFlexibleBox can override it.

No new tests, this is just a refactoring.

* rendering/RenderBlock.cpp:
(WebCore::RenderBlock::paintContents): Move some code out of paintChildren into paintContents.
(WebCore::RenderBlock::paintChildren): Make virtual and just have it be a simple for loop.
(WebCore::RenderBlock::paintChild): Move code out of paintChildren so subclasses can reuse.
* rendering/RenderBlock.h:
(RenderBlock): Make paintChildren virtual and protected so sub classes can call paintChild.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@123524 268f45cc-cd09-0410-ab3c-d52691b4dbfc