Add 4.20 as a GLSL output version.

GLSL 4.20 adds support for image functions which will be used in forthcoming changes.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4833

Change-Id: I8cab511d95f04b45faaf916628148117b8be2128
Reviewed-on: https://skia-review.googlesource.com/4833
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/gpu/glsl/GrGLSL.cpp b/src/gpu/glsl/GrGLSL.cpp
index bec4784..69f046c 100644
--- a/src/gpu/glsl/GrGLSL.cpp
+++ b/src/gpu/glsl/GrGLSL.cpp
@@ -18,6 +18,7 @@
         case k150_GrGLSLGeneration:
         case k330_GrGLSLGeneration:
         case k400_GrGLSLGeneration:
+        case k420_GrGLSLGeneration:
         case k310es_GrGLSLGeneration:
         case k320es_GrGLSLGeneration:
             return true;