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

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 587b0b98aa433bf787843ce51172501b1138ac36..c9b3f39c8d391f1482cb29b742ea7eb91f8414a0 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -948,13 +948,13 @@ public:
             const GrTexture* backgroundTex = drawEffect.castEffect<XferEffect>().backgroundAccess().getTexture();
             const char* dstColor;
             if (backgroundTex) {
-                const char* bgCoords;
+                SkString bgCoords;
                 GrSLType bgCoordsType = fBackgroundEffectMatrix.emitCode(builder, key, &bgCoords, NULL, "BG");
                 dstColor = "bgColor";
                 builder->fsCodeAppendf("\t\tvec4 %s = ", dstColor);
                 builder->appendTextureLookup(GrGLShaderBuilder::kFragment_ShaderType,
                                              samplers[0],
-                                             bgCoords,
+                                             bgCoords.c_str(),
                                              bgCoordsType);
                 builder->fsCodeAppendf(";\n");
             } else {