Potentially optimize some GrGLEffects for known input color values

src/gpu/gl/GrGLShaderBuilder.h

Index: src/gpu/gl/GrGLShaderBuilder.h
diff --git a/src/gpu/gl/GrGLShaderBuilder.h b/src/gpu/gl/GrGLShaderBuilder.h
index bcbff05bd9fb4feed04fe9e3d2f23333bcd2305c..26d5e1ad6323e871d65e3ce6d2a23f98a433d554 100644
--- a/src/gpu/gl/GrGLShaderBuilder.h
+++ b/src/gpu/gl/GrGLShaderBuilder.h
@@ -90,7 +90,7 @@ public:
         always a vec4. modulation and the swizzle specified by TextureSampler must both be vec4 or
         float. If modulation is "" or NULL it this function acts as though appendTextureLookup were
         called. */
-    void fsAppendTextureLookupAndModulate(const char* modulation,
+    void fsAppendTextureLookupAndModulate(const GrGLSLExpr4& modulation,
                                           const TextureSampler&,
                                           const char* coordName,
                                           GrSLType coordType = kVec2f_GrSLType);