Potentially optimize some GrGLEffects for known input color values

src/effects/SkLumaColorFilter.cpp

Index: src/effects/SkLumaColorFilter.cpp
diff --git a/src/effects/SkLumaColorFilter.cpp b/src/effects/SkLumaColorFilter.cpp
index fd4826b5a9b92c8fb6a31472a7fe6cb799960ae3..38aaa86d00551e8918a9c19b4ea33500726a517b 100644
--- a/src/effects/SkLumaColorFilter.cpp
+++ b/src/effects/SkLumaColorFilter.cpp
@@ -94,18 +94,14 @@ public:
                               const GrDrawEffect&,
                               EffectKey,
                               const char* outputColor,
-                              const char* inputColor,
+                              const GrGLSLExpr4& inputColor,
                               const TransformedCoordsArray&,
                               const TextureSamplerArray&) SK_OVERRIDE {
-            if (NULL == inputColor) {
-                inputColor = "vec4(1)";
-            }
-
             builder->fsCodeAppendf("\tfloat luma = dot(vec3(%f, %f, %f), %s.rgb);\n",
                                    SK_ITU_BT709_LUM_COEFF_R,
                                    SK_ITU_BT709_LUM_COEFF_G,
                                    SK_ITU_BT709_LUM_COEFF_B,
-                                   inputColor);
+                                   inputColor.c_str());
             builder->fsCodeAppendf("\t%s = vec4(0, 0, 0, luma);\n",
                                    outputColor);