Adding feature flag that forces highp in fragment shader to mediump. It
is disabled by default. You can enable it for experiment.

Bug: b/184850002
Change-Id: I73980be4ad160eb1b17a3a3ecfc09f2e4aeb468c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2815240Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>