Basically outside TCompile::compile(), the global parse context is
invalid, and should never be queried.

BUG=angle:568
TEST=webgl conformance tests, no crash

Change-Id: I5573ce2bf3bf838ab24f59dda00948f60a0b023d
Reviewed-on: https://chromium-review.googlesource.com/197178Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>