Makes it an error to access gl_FragDepthEXT  in #version 300 es shader.

TODO:
Lacks the feature to make "#extension GL_EXT_frag_depth : require" an
error for #version 300 es.

BUG=angleproject:1102
TEST=angle_unittest

Change-Id: Ic313a0e1ed6369550d53885290f300fab1aaf304
Reviewed-on: https://chromium-review.googlesource.com/287570Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>