Reviewed by Darin Adler.

        Protect JSC from WebCore executing JS during JS wrapper finalization
        https://bugs.webkit.org/show_bug.cgi?id=60672
        <rdar://problem/9350997>

        Detect when we're trying to execute JS during GC and prevent the
        execution from happening.  We also assert that this isn't happening
        as it implies incorrect behaviour of an object's destructor.

        * JavaScriptCore.exp:
        * heap/Heap.cpp:
        * heap/Heap.h:
        (JSC::Heap::isBusy):
        * interpreter/Interpreter.cpp:
        (JSC::Interpreter::execute):
        (JSC::Interpreter::executeCall):
        (JSC::Interpreter::executeConstruct):
        * runtime/JSGlobalData.h:
        (JSC::JSGlobalData::isCollectorBusy):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@86300 268f45cc-cd09-0410-ab3c-d52691b4dbfc