Refactor validating jump statements out of glslang.y Move the logic to ParseContext. This will simplify changing the logic for the break statement that is needed to implement switch. BUG=angle:921, angle:911 TEST=WebGL conformance tests Change-Id: Ib8bbc7075c05d345e90377202b5446fb3307f94c Reviewed-on: https://chromium-review.googlesource.com/251522 Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Zhenyao Mo <zmo@chromium.org>