Abstracted diffuse color map out of SkLightingShaderImpl into a generic SkShader

src/core/SkNormalSource.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkError.h"
 #include "SkErrorInternals.h"
 #include "SkLightingShader.h"
@@ skipping 163 matching lines @@
 
     SkMatrix fInvCTM;
 };
 
 sk_sp<GrFragmentProcessor> NormalMapSourceImpl::asFragmentProcessor(
                                                      GrContext *context,
                                                      const SkMatrix &viewM,
                                                      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);
 }
 
 #endif // SK_SUPPORT_GPU
 
 ////////////////////////////////////////////////////////////////////////////
 
 NormalMapSourceImpl::Provider::Provider(const NormalMapSourceImpl& source,
                                         SkShader::Context* mapContext)
@@ skipping 117 matching lines @@
     return sk_make_sp<NormalMapSourceImpl>(std::move(map), invCTM);
 }
 
 ////////////////////////////////////////////////////////////////////////////
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkNormalSource)
     SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(NormalMapSourceImpl)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
 
 ////////////////////////////////////////////////////////////////////////////