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.

Reland of: https://chromium-review.googlesource.com/#/c/287570

BUG=angleproject:1102
TEST=angle_unittest

Change-Id: I064d918d65f37539cb1e14f12173ca5591a4ea3f
Reviewed-on: https://chromium-review.googlesource.com/301711Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>