Add SkCachedData and use it for SkMipMap

gyp/tests.gypi

 # Common gypi for unit tests.
 {
   'include_dirs': [
     '../src/core',
     '../src/effects',
     '../src/image',
     '../src/lazy',
     '../src/images',
     '../src/pathops',
     '../src/pdf',
@@ skipping 113 matching lines @@
     '../tests/ImageNewShaderTest.cpp',
     '../tests/InfRectTest.cpp',
     '../tests/InterpolatorTest.cpp',
     '../tests/JpegTest.cpp',
     '../tests/KtxTest.cpp',
     '../tests/LListTest.cpp',
     '../tests/LayerDrawLooperTest.cpp',
     '../tests/LayerRasterizerTest.cpp',
     '../tests/MD5Test.cpp',
     '../tests/MallocPixelRefTest.cpp',
+    '../tests/MaskCacheTest.cpp',
     '../tests/MathTest.cpp',
     '../tests/Matrix44Test.cpp',
     '../tests/MatrixClipCollapseTest.cpp',
     '../tests/MatrixTest.cpp',
     '../tests/MemoryTest.cpp',
     '../tests/MemsetTest.cpp',
     '../tests/MessageBusTest.cpp',
     '../tests/MetaDataTest.cpp',
     '../tests/MiniDataTest.cpp',
     '../tests/MipMapTest.cpp',
@@ skipping 77 matching lines @@
     '../src/utils/debugger/SkObjectParser.h',
     '../src/utils/debugger/SkObjectParser.cpp',
 
     '../tests/PipeTest.cpp',
     '../src/pipe/utils/SamplePipeControllers.cpp',
 
     '../tests/TDStackNesterTest.cpp',
     '../experimental/PdfViewer/src/SkTDStackNester.h',
   ],
 }