Adds precision emulation and WebGL 2 and GLES3 specs as options. The
example command line is also fixed.

Change-Id: Ia45eca1815724a63b26662ee0cadecf9dfea51f9
Reviewed-on: https://chromium-review.googlesource.com/236793Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>