Convert GrClip to an abstract base class

tests/ClipBoundsTest.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Test.h"
 // This is a GR test
 #if SK_SUPPORT_GPU
@@ skipping 22 matching lines @@
 
     stack.getConservativeBounds(0, 0, kXSize, kYSize,
                                 &devStackBounds,
                                 &isIntersectionOfRects);
 
     // make sure that the SkClipStack is behaving itself
     REPORTER_ASSERT(reporter, screen == devStackBounds);
     REPORTER_ASSERT(reporter, isIntersectionOfRects);
 
     // wrap the SkClipStack in a GrClip
-    GrClip clipData;
-    clipData.setClipStack(&stack);
+    GrClipStackClip clipData(&stack);
 
     SkIRect devGrClipBound;
     clipData.getConservativeBounds(kXSize, kYSize,
                                    &devGrClipBound,
                                    &isIntersectionOfRects);
 
     // make sure that GrClip is behaving itself
     REPORTER_ASSERT(reporter, intScreen == devGrClipBound);
     REPORTER_ASSERT(reporter, isIntersectionOfRects);
 }
 
 #endif