Convert GrClip to an abstract base class

src/effects/SkAlphaThresholdFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAlphaThresholdFilter.h"
 
 #include "SkBitmap.h"
@@ skipping 96 matching lines @@
                                                              config));
     if (!drawContext) {
         return nullptr;
     }
 
     GrPaint grPaint;
     grPaint.setPorterDuffXPFactory(SkXfermode::kSrc_Mode);
     SkRegion::Iterator iter(fRegion);
     drawContext->clear(nullptr, 0x0, true);
 
-    GrClip clip(SkRect::Make(SkIRect::MakeWH(bounds.width(), bounds.height())));
+    GrFixedClip clip(SkIRect::MakeWH(bounds.width(), bounds.height()));
     while (!iter.done()) {
         SkRect rect = SkRect::Make(iter.rect());
         drawContext->drawRect(clip, grPaint, inMatrix, rect);
         iter.next();
     }
 
     return drawContext->asTexture();
 }
 #endif
 
@@ skipping 131 matching lines @@
                                           dst);
 }
 
 #ifndef SK_IGNORE_TO_STRING
 void SkAlphaThresholdFilterImpl::toString(SkString* str) const {
     str->appendf("SkAlphaThresholdImageFilter: (");
     str->appendf("inner: %f outer: %f", fInnerThreshold, fOuterThreshold);
     str->append(")");
 }
 #endif