Communicating between Lightning component and VF Page
Finallyyyyyyyy I figured out !!!
I had set development mode on in my user profile**** because of which I was getting the following error.
Uncaught DOMException: Blocked a frame with origin "https://praowin-dev-ed--c.gus.visual.force.com" from accessing a cross-origin frame. at EditAreaLoader.add_event
As soon as I deactivated the development mode and lost access to the VF code editor, the window.postmessage was successfully posting as expected.