Fix bug in erosion filtering

src/effects/SkMorphologyImageFilter.cpp

Index: src/effects/SkMorphologyImageFilter.cpp
===================================================================
--- src/effects/SkMorphologyImageFilter.cpp	(revision 11994)
+++ src/effects/SkMorphologyImageFilter.cpp	(working copy)
@@ -507,7 +507,9 @@
                               morphType, Gr1DKernelEffect::kX_Direction);
         SkIRect clearRect = SkIRect::MakeXYWH(dstRect.fLeft, dstRect.fBottom,
                                               dstRect.width(), radius.fHeight);
-        context->clear(&clearRect, 0x0);
+        context->clear(&clearRect, GrMorphologyEffect::kErode_MorphologyType == morphType ? 
+                                   SK_ColorWHITE : 
+                                   SK_ColorTRANSPARENT);
         src.reset(ast.detach());
         srcRect = dstRect;
     }