Add comment to clarify isStackingContext() in RenderLayer.h
https://bugs.webkit.org/show_bug.cgi?id=113909

Reviewed by Simon Fraser.

It was not obvious how the implementation of isStackingContext()
was correct, a comment is appropriate here for anyone who reads
this code in the future.

Just a change in comment, no test needed.

* rendering/RenderLayer.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147600 268f45cc-cd09-0410-ab3c-d52691b4dbfc