Make JSCell::toBoolean non-virtual

Make JSCell::toBoolean non-virtual
https://bugs.webkit.org/show_bug.cgi?id=67727

Reviewed by Geoffrey Garen.

JSCell::toBoolean now manually performs the toBoolean check for objects and strings (where
before it was simply virtual and would crash if its implementation was called).
Its descendants in JSObject and JSString have also been made non-virtual.  JSCell now
explicitly covers all cases of toBoolean, so having a virtual implementation of
JSCell::toBoolean is no longer necessary.  This is part of a larger process of un-virtualizing JSCell.

* JavaScriptCore.exp:
* JavaScriptCore.vcproj/JavaScriptCore/JavaScriptCore.def:
* runtime/JSCell.cpp:
* runtime/JSCell.h:
* runtime/JSNotAnObject.cpp:
* runtime/JSNotAnObject.h:
* runtime/JSObject.h:
* runtime/JSString.h:
(JSC::JSCell::toBoolean):
(JSC::JSValue::toBoolean):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96045 268f45cc-cd09-0410-ab3c-d52691b4dbfc