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

src/effects/gradients/SkRadialGradient.cpp

Index: src/effects/gradients/SkRadialGradient.cpp
diff --git a/src/effects/gradients/SkRadialGradient.cpp b/src/effects/gradients/SkRadialGradient.cpp
index ca659699950f791935605bb50309ee4d3335aa8f..9fcce68fa3a1b9e2ccfb91786388e61e9face951 100644
--- a/src/effects/gradients/SkRadialGradient.cpp
+++ b/src/effects/gradients/SkRadialGradient.cpp
@@ -559,7 +559,7 @@ void GrGLRadialGradient::emitCode(GrGLShaderBuilder* builder,
                                   const char* inputColor,
                                   const TextureSamplerArray& samplers) {
     this->emitYCoordUniform(builder);
-    const char* coords;
+    SkString coords;
     this->setupMatrix(builder, key, &coords);
     SkString t("length(");
     t.append(coords);