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 40 matching lines @@
     '../tests/AtomicTest.cpp',
     '../tests/BitSetTest.cpp',
     '../tests/BitmapCopyTest.cpp',
     '../tests/BitmapGetColorTest.cpp',
     '../tests/BitmapHasherTest.cpp',
     '../tests/BitmapHeapTest.cpp',
     '../tests/BitmapTest.cpp',
     '../tests/BlendTest.cpp',
     '../tests/BlitRowTest.cpp',
     '../tests/BlurTest.cpp',
+    '../tests/CachedDataTest.cpp',
     '../tests/CachedDecodingPixelRefTest.cpp',
     '../tests/CanvasStateHelpers.cpp',
     '../tests/CanvasStateTest.cpp',
     '../tests/CanvasTest.cpp',
     '../tests/ChecksumTest.cpp',
     '../tests/ClampRangeTest.cpp',
     '../tests/ClipCacheTest.cpp',
     '../tests/ClipCubicTest.cpp',
     '../tests/ClipStackTest.cpp',
     '../tests/ClipperTest.cpp',
@@ skipping 146 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',
   ],
 }