Convert GrClip to an abstract base class

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index a4876c800899b98c436cd687e85adb1a6b7e2f34..b4848923ea07e82a263a31a3daa4cab99692ec5c 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -114,7 +114,7 @@ sk_sp<GrTexture> SkAlphaThresholdFilterImpl::createMaskTexture(GrContext* contex
     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);