FloatingObjects should manage cleaning it's line box tree pointers itself

FloatingObjects should manage cleaning it's line box tree pointers itself
https://bugs.webkit.org/show_bug.cgi?id=120692

Reviewed by David Hyatt.

This is another step in properly encapsulating FloatingObjects.
Instead of having RenderBlock walk and clear the line box tree
pointers, create a method for the behavior, and have RenderBlock call
that.

In addtion, add a proper destructor to FloatingObjects, so that
RenderBlock does not have to explicitly delete the set in
FloatingObjects.

And as a bonus, fix the ordering of an if to avoid the expensive
descendantChild check.

This is a port of a Blink patch by Eric Seidel.

No new tests, no behavior change.

* rendering/RenderBlock.cpp:
(WebCore::RenderBlock::~RenderBlock):
(WebCore::RenderBlock::deleteLineBoxTree):
(WebCore::RenderBlock::repaintOverhangingFloats):
(WebCore::RenderBlock::FloatingObjects::~FloatingObjects):
(WebCore::RenderBlock::FloatingObjects::clearLineBoxTreePointers):
(WebCore::RenderBlock::FloatingObjects::clear):
* rendering/RenderBlock.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@155065 268f45cc-cd09-0410-ab3c-d52691b4dbfc