Implement canComputeFastBounds() for image filters.

include/effects/SkRectShaderImageFilter.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkRectShaderImageFilter_DEFINED
 #define SkRectShaderImageFilter_DEFINED
 
@@ skipping 12 matching lines @@
      *               ref'ed by the new image filter.
      *  @param rect  Rectangle of output pixels in which to apply the shader.
      *               If NULL or a given crop edge is not specified, the source
      *               primitive's bounds are used instead.
      */
     SK_ATTR_DEPRECATED("use Create(SkShader*, const CropRect*)")
     static SkRectShaderImageFilter* Create(SkShader* s, const SkRect& rect);
 
     static SkRectShaderImageFilter* Create(SkShader* s, const CropRect* rect = NULL);
     virtual ~SkRectShaderImageFilter();
+    bool affectsTransparentBlack() const override;
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkRectShaderImageFilter)
 
 protected:
     void flatten(SkWriteBuffer&) const override;
 
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const Context&,
                                SkBitmap* result, SkIPoint* loc) const override;
 
 private:
     SkRectShaderImageFilter(SkShader* s, const CropRect* rect);
     SkShader*  fShader;
 
     typedef SkImageFilter INHERITED;
 };
 
 #endif