Make SkGLContext lifetime more well-defined

include/gpu/gl/SkGLContext.h

Index: include/gpu/gl/SkGLContext.h
diff --git a/include/gpu/gl/SkGLContext.h b/include/gpu/gl/SkGLContext.h
index 3420a47973228594d4d1f8f6fc24cca9df685f3c..7edfa730aa746ac33e3366e327645df9f38aeea7 100644
--- a/include/gpu/gl/SkGLContext.h
+++ b/include/gpu/gl/SkGLContext.h
@@ -17,9 +17,9 @@
  * This class is intended for Skia's testing needs and not for general
  * use.
  */
-class SK_API SkGLContext : public SkRefCnt {
+class SK_API SkGLContext : public SkNoncopyable {
 public:
-    ~SkGLContext() override;
+    virtual ~SkGLContext();
 
     bool isValid() const { return NULL != gl(); }
 
@@ -106,8 +106,6 @@ private:
     SkAutoTUnref<const GrGLInterface> fGL;
 
     friend class GLFenceSync;  // For onPlatformGetProcAddress.
-
-    typedef SkRefCnt INHERITED;
 };
 
 /** Creates platform-dependent GL context object