Update SkImageFilter's cache to handle SkSpecialImages and add unit test

gyp/core.gypi

 # 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 this gypi to include all 'core' files
 # The parent gyp/gypi file must define
 #       'skia_src_path'     e.g. skia/trunk/src
 #       'skia_include_path' e.g. skia/trunk/include
 #
 # The skia build defines these in common_variables.gypi
@@ skipping 122 matching lines @@
         '<(skia_src_path)/core/SkGeometry.h',
         '<(skia_src_path)/core/SkGlobalInitialization_core.cpp',
         '<(skia_src_path)/core/SkGlyph.h',
         '<(skia_src_path)/core/SkGlyphCache.cpp',
         '<(skia_src_path)/core/SkGlyphCache.h',
         '<(skia_src_path)/core/SkGlyphCache_Globals.h',
         '<(skia_src_path)/core/SkGraphics.cpp',
         '<(skia_src_path)/core/SkHalf.cpp',
         '<(skia_src_path)/core/SkHalf.h',
         '<(skia_src_path)/core/SkImageFilter.cpp',
+        '<(skia_src_path)/core/SkImageFilterCacheKey.h',
         '<(skia_src_path)/core/SkImageInfo.cpp',
         '<(skia_src_path)/core/SkImageCacherator.h',
         '<(skia_src_path)/core/SkImageCacherator.cpp',
         '<(skia_src_path)/core/SkImageGenerator.cpp',
         '<(skia_src_path)/core/SkImageGeneratorPriv.h',
         '<(skia_src_path)/core/SkLayerInfo.h',
         '<(skia_src_path)/core/SkLight.h',
         '<(skia_src_path)/core/SkLightingShader.h',
         '<(skia_src_path)/core/SkLightingShader.cpp',
         '<(skia_src_path)/core/SkLinearBitmapPipeline.cpp',
@@ skipping 326 matching lines @@
         '<(skia_src_path)/pathops/SkPathOpsLine.h',
         '<(skia_src_path)/pathops/SkPathOpsPoint.h',
         '<(skia_src_path)/pathops/SkPathOpsQuad.h',
         '<(skia_src_path)/pathops/SkPathOpsRect.h',
         '<(skia_src_path)/pathops/SkPathOpsTSect.h',
         '<(skia_src_path)/pathops/SkPathOpsTypes.h',
         '<(skia_src_path)/pathops/SkPathWriter.h',
         '<(skia_src_path)/pathops/SkReduceOrder.h',
     ],
 }