Fix an issue with separate invariant statement.

Basically we end up with an extra "invariant xxx ;;" on GL backend.

This is not handled correctly by Mac drivers and also out shader translator.

BUG=angle:776
TEST=webgl conformance on mac/windows

Change-Id: If32867efba64ab0d3e6e690155790b90d736439f
Reviewed-on: https://chromium-review.googlesource.com/225780
Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>