ES31: Add std430 support for OpenGL backend

BUG=angleproject:1920
TEST=angle_unittests

Change-Id: Ie8e171abec053c2c5dca93d6e79db534f74520e7
Reviewed-on: https://chromium-review.googlesource.com/737532
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/compiler/translator/OutputGLSLBase.cpp b/src/compiler/translator/OutputGLSLBase.cpp
index 642a371..bc4867a 100644
--- a/src/compiler/translator/OutputGLSLBase.cpp
+++ b/src/compiler/translator/OutputGLSLBase.cpp
@@ -1186,6 +1186,10 @@
             out << "std140";
             break;
 
+        case EbsStd430:
+            out << "std430";
+            break;
+
         default:
             UNREACHABLE();
             break;