Reviewed by Darin Adler.

        WebKit crashes at DebuggerCallFrame::functionName() if m_callFrame is the top global callframe.
        https://bugs.webkit.org/show_bug.cgi?id=38535

        WebScriptDebugger in WebKit has empty implementations for willExecuteProgram and didExecuteProgram.  As a result,
        if the top call frame is from a program, WebKitScriptDebugger doesn't record that callframe as the top frame, and
        WebScriptDebugger's callframe stack is wrong from this point.  That could cause crash if we trying to access the top
        call frame from this stack when an exception throws because the saved top frame could be invalid.

        To fix that, we need to maintain the call frame stack in willExecuteProgram and didExecuteProgram, as we did in
        callEvent and returnEvent.

        * WebView/WebScriptDebugger.mm:
        (WebScriptDebugger::willExecuteProgram):
        (WebScriptDebugger::didExecuteProgram):

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