Most code using the translator already enables highp with the resources
flag when a shader spec that accepts ESSL3 is used, but for example the
shader_translator utility doesn't. This fix makes sure that highp is
always enabled when a fragment shader written in ESSL3 or newer is being
compiled. This will make shader_translator easier to use for testing
ESSL3 shaders.

BUG=541550
TEST=angle_unittests

Change-Id: Ia1911677c55f3c5d921829a8cbb808847ac8b636
Reviewed-on: https://chromium-review.googlesource.com/305190
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>