Fix case where GrGLEffectMatrix gives back a dangling ptr for the coords var name.

src/effects/SkLightingImageFilter.cpp

Index: src/effects/SkLightingImageFilter.cpp
diff --git a/src/effects/SkLightingImageFilter.cpp b/src/effects/SkLightingImageFilter.cpp
index e71df66e6c6888432d1a1094b20f140a2c058112..f907a199b810b44a8019bb3fd1f7b20f9b1e5306 100644
--- a/src/effects/SkLightingImageFilter.cpp
+++ b/src/effects/SkLightingImageFilter.cpp
@@ -1204,7 +1204,7 @@ void GrGLLightingEffect::emitCode(GrGLShaderBuilder* builder,
                                   const char* outputColor,
                                   const char* inputColor,
                                   const TextureSamplerArray& samplers) {
-    const char* coords;
+    SkString coords;
     fEffectMatrix.emitCodeMakeFSCoords2D(builder, key, &coords);
 
     fImageIncrementUni = builder->addUniform(GrGLShaderBuilder::kFragment_ShaderType,
@@ -1267,7 +1267,7 @@ void GrGLLightingEffect::emitCode(GrGLShaderBuilder* builder,
                           interiorNormalBody.c_str(),
                           &interiorNormalName);
 
-    builder->fsCodeAppendf("\t\tvec2 coord = %s;\n", coords);
+    builder->fsCodeAppendf("\t\tvec2 coord = %s;\n", coords.c_str());
     builder->fsCodeAppend("\t\tfloat m[9];\n");
 
     const char* imgInc = builder->getUniformCStr(fImageIncrementUni);