Otherwise we may run into a name conflict issue if the shader code is
run through the translator twice, since neither user-defined function
names or internal function names are currently being prefixed in GLSL
output. This could be fixed by prefixing user-defined variables in
GLSL output, but this solution is much simpler.

BUG=angleproject:1966
TEST=angle_end2end_tests, WebGL conformance tests

Change-Id: I15c8de5a0a5e596fafe9f55e8d370345c790d5cc
Reviewed-on: https://chromium-review.googlesource.com/509688
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>