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/251522Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>