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

src/effects/gradients/SkSweepGradient.cpp

Index: src/effects/gradients/SkSweepGradient.cpp
diff --git a/src/effects/gradients/SkSweepGradient.cpp b/src/effects/gradients/SkSweepGradient.cpp
index f975a1887ead036816bd2f190fd36cb8f76a6c5e..2af20b0bf6ce84b12350df9a4966df2a1be051b6 100644
--- a/src/effects/gradients/SkSweepGradient.cpp
+++ b/src/effects/gradients/SkSweepGradient.cpp
@@ -473,10 +473,10 @@ void GrGLSweepGradient::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.printf("atan(- %s.y, - %s.x) * 0.1591549430918 + 0.5", coords, coords);
+    t.printf("atan(- %s.y, - %s.x) * 0.1591549430918 + 0.5", coords.c_str(), coords.c_str());
     this->emitColorLookup(builder, t.c_str(), outputColor, inputColor, samplers[0]);
 }