Important note: it appeared that this trick is not working in Unity 2017.2 and higher. Since currently my primary version is 2017.1.3 I have no time to experiment and find solution for the mentioned versions 😕
GrabPass is probably the easiest way to get the content of the screen and pass it to the shader automagically. Later you can use this texture to make some nifty effects like distortion and reflection.
But GrabPass has a huge fault: it breaks the dynamic batching 💔