Abstracted diffuse color map out of SkLightingShaderImpl into a generic SkShader

src/core/SkNormalSource.cpp

Index: src/core/SkNormalSource.cpp
diff --git a/src/core/SkNormalSource.cpp b/src/core/SkNormalSource.cpp
index 52bb4adaa70092504a58bc4f17ecd825f6b4988a..a69367921263ccdd8d1f2f793d35bc3923c6deef 100644
--- a/src/core/SkNormalSource.cpp
+++ b/src/core/SkNormalSource.cpp
@@ -181,9 +181,11 @@ sk_sp<GrFragmentProcessor> NormalMapSourceImpl::asFragmentProcessor(
                                                      const SkMatrix *localMatrix,
                                                      SkFilterQuality filterQuality,
                                                      SkSourceGammaTreatment gammaTreatment) const {
-
     sk_sp<GrFragmentProcessor> mapFP = fMapShader->asFragmentProcessor(context, viewM,
             localMatrix, filterQuality, gammaTreatment);
+    if (!mapFP) {
+        return nullptr;
+    }
 
     return sk_make_sp<NormalMapFP>(std::move(mapFP), fInvCTM);
 }