Reviewed by Geoff Garen.

        - https://bugs.webkit.org/show_bug.cgi?id=21214
          work on getting rid of ExecState

        Replaced the m_prev field of ExecState with a bit in the
        call frame pointer to indicate "host" call frames.

        * VM/Machine.cpp:
        (JSC::makeHostCallFramePointer): Added. Sets low bit.
        (JSC::isHostCallFrame): Added. Checks low bit.
        (JSC::stripHostCallFrameBit): Added. Clears low bit.
        (JSC::Machine::unwindCallFrame): Replaced null check that was
        formerly used to detect host call frames with an isHostCallFrame check.
        (JSC::Machine::execute): Pass in a host call frame pointer rather than
        always passing 0 when starting execution from the host. This allows us
        to follow the entire call frame pointer chain when desired, or to stop
        at the host calls when that's desired.
        (JSC::Machine::privateExecute): Replaced null check that was
        formerly used to detect host call frames with an isHostCallFrame check.
        (JSC::Machine::retrieveCaller): Ditto.
        (JSC::Machine::retrieveLastCaller): Ditto.
        (JSC::Machine::callFrame): Removed the code to walk up m_prev pointers
        and replaced it with code that uses the caller pointer and uses the
        stripHostCallFrameBit function.

        * kjs/ExecState.cpp: Removed m_prev.
        * kjs/ExecState.h: Ditto.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37125 268f45cc-cd09-0410-ab3c-d52691b4dbfc