Add setter for GrGLSLShaderVar layout qualifier

BUG=skia:
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Review URL: https://codereview.chromium.org/1611423002

src/gpu/glsl/GrGLSLShaderVar.h

@@ -129,6 +129,13 @@ public:
         fUseUniformFloatArrays = useUniformFloatArrays;
     }
 
+    /**
+     * Set the layout qualifier
+     */
+    void setLayoutQualifier(const char* layoutQualifier) {
+        fLayoutQualifier = layoutQualifier;
+    }
+
     /**
      * Write a declaration of this variable to out.
      */