Handy code for debug/experiment that forces highp in fragment shader to
mediump.

Bug: b/173140243
Change-Id: I1841b67862343df8a12bf302066d7eefd7709604
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2537072
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>