Remove webgl_ prefix from emulated function names The prefix is unnecessary now that user-defined names are prefixed in both GLSL and HLSL output. Removing the prefix makes compiler output a bit simpler to read. BUG=angleproject:2038 TEST=angle_unittests Change-Id: I9ffc508f50d6146a2d85798875c88e2c385b83fe Reviewed-on: https://chromium-review.googlesource.com/508730 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>