Refactor conditions for setCompositingLayersNeedRebuild in RenderLayer::styleChanged
https://bugs.webkit.org/show_bug.cgi?id=112606

Reviewed by Simon Fraser.

To fix bug 109098 [1], we will need to add more conditions for calling
RenderLayerCompositor::setCompositingLayersNeedRebuild in RenderLayer::styleChanged.
Adding more non-trivial conditions will make this block of code even harder to follow.
This patch refactors the current conditions into functions:
    needsCompositingLayersRebuiltForClip
    needsCompositingLayersRebuiltForOverflow

In bug 109098 [1], I'm planning to add:
    needsCompositingLayersRebuiltForFilters

[1]: https://bugs.webkit.org/show_bug.cgi?id=109098

No new tests. Just refactoring.

* rendering/RenderLayer.cpp:
(WebCore):
(WebCore::RenderLayer::needsCompositingLayersRebuiltForClip):
(WebCore::RenderLayer::needsCompositingLayersRebuiltForOverflow):
    This is a method and not a file static function because it needs to look at the
    RenderLayer's stackingContainer(), which is a private method.
(WebCore::RenderLayer::styleChanged):
* rendering/RenderLayer.h:
(RenderLayer):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146213 268f45cc-cd09-0410-ab3c-d52691b4dbfc