[CSSRegions] Improve hit testing for empty regions
https://bugs.webkit.org/show_bug.cgi?id=115534

Reviewed by David Hyatt.

As a better follow up after WebKit bug https://bugs.webkit.org/show_bug.cgi?id=107752,
we can improve the code by not doing hit testing on the render flow thread object when
we are trying to hit test the render flow thread background. In this case, we should just bail out.
No new tests, covered by existing regions tests.

* rendering/RenderFlowThread.cpp:
(WebCore::RenderFlowThread::nodeAtPoint):
(WebCore):
(WebCore::RenderFlowThread::hitTestFlowThreadPortionInRegion):
* rendering/RenderFlowThread.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150078 268f45cc-cd09-0410-ab3c-d52691b4dbfc