Not implemented: isContextLost is not a function

[kudlav]

Hi,
I have tried to use this bridge to run TF.js with WebGL backend, but it seems that this bridge does not implement isContextLost()
see https://developer.mozilla.org/en-US/docs/Web/API/WebGLRenderingContext/isContextLost

As a workaround I've implemented simple function that returns always false. It's not crashing but TF.js WebGL backend still does not work with this bridge.