Make RunLoop ref-counted
https://bugs.webkit.org/show_bug.cgi?id=115384

Reviewed by Sam Weinig.

This is a step towards creating a shared base class that both RunLoop and WorkQueue will derive from.

* platform/RunLoop.cpp:
(WebCore::RunLoop::current):
(WebCore::RunLoop::~RunLoop):
(WebCore):
* platform/RunLoop.h:
(RunLoop):
* platform/cf/RunLoopCF.cpp:
(WebCore::RunLoop::initializeMainRunLoop):
(WebCore::RunLoop::current):
(WebCore::RunLoop::~RunLoop):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149338 268f45cc-cd09-0410-ab3c-d52691b4dbfc