Reviewed by Darin Adler.

        <rdar://problem/4949002> JSGlobalContextCreate can cause crashes because it passes a NULL JSContextRef to the globalObjectClass's initialize callback

        JSCallbackObject now tracks whether it was constructed with a null ExecState.  This will happen when the object is being used as the global object,
        as the Interpreter needs to be created after the global object.  In this situation the initialization is deferred until after the Interpreter's
        ExecState is available to be passed down to the initialize callbacks.

        * API/JSCallbackObject.cpp:
        (KJS::JSCallbackObject::init): Track whether we successfully initialized.
        (KJS::JSCallbackObject::initializeIfNeeded): Attempt to initialize with the new ExecState.
        * API/JSCallbackObject.h:
        * API/JSContextRef.cpp:
        (JSGlobalContextCreate): Initialize the JSCallbackObject with the Interpreter's ExecState.
        * API/testapi.c:
        (testInitializeOfGlobalObjectClassHasNonNullContext):
        (main): Verify that the context passed to the initialize callback is non-null.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@25257 268f45cc-cd09-0410-ab3c-d52691b4dbfc