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

R=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://chromiumcodereview.appspot.com/23706003

git-svn-id: http://skia.googlecode.com/svn/trunk@10982 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/effects/gradients/SkGradientShaderPriv.h b/src/effects/gradients/SkGradientShaderPriv.h
index 31cc9f2..8e93604 100644
--- a/src/effects/gradients/SkGradientShaderPriv.h
+++ b/src/effects/gradients/SkGradientShaderPriv.h
@@ -317,8 +317,8 @@
      */
     void setupMatrix(GrGLShaderBuilder* builder,
                      EffectKey key,
-                     const char** fsCoordName,
-                     const char** vsVaryingName = NULL,
+                     SkString* fsCoordName,
+                     SkString* vsVaryingName = NULL,
                      GrSLType* vsVaryingType = NULL);
 
     // Emits the uniform used as the y-coord to texture samples in derived classes. Subclasses