Implement gl_FragDepth for GLES SL 3.0

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/301711
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/compiler/translator/OutputHLSL.cpp b/src/compiler/translator/OutputHLSL.cpp
index d6b9a55..117c34a 100644
--- a/src/compiler/translator/OutputHLSL.cpp
+++ b/src/compiler/translator/OutputHLSL.cpp
@@ -1386,7 +1386,7 @@
             mUsesInstanceID = true;
             out << name;
         }
-        else if (name == "gl_FragDepthEXT")
+        else if (name == "gl_FragDepthEXT" || name == "gl_FragDepth")
         {
             mUsesFragDepth = true;
             out << "gl_Depth";