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

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index 2f56cb49ca21c2b4443663a5259def64bf31f24d..76a44a26955b719bef8518281500338487d4f420 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -525,7 +525,7 @@ void GrGLLinearGradient::emitCode(GrGLShaderBuilder* builder,
                                   const char* inputColor,
                                   const TextureSamplerArray& samplers) {
     this->emitYCoordUniform(builder);
-    const char* coords;
+    SkString coords;
     this->setupMatrix(builder, key, &coords);
     SkString t;
     t.append(coords);