I think the people behind this come from a OpenGL-ES / WebGL background. Coaxing data back out of OpenGL-ES / WebGL is a bit tricker compared to original OpenGL. But only a little bit (you need a proxy FBO to retrieve texture data; you can't write to buffer objects with shaders in ES/WebGL so hooking glBufferData suffices).
I don't think it will be difficult. after all chromium and Angle are opensource, you can hack chromium source code to get everything. plus, javascript is already kinda open to steal. same to android. it is open.
on ios, it's harder. but shouldn't ios already encrypt all its apps?