Add SkCachedData and use it for SkMipMap

This reverts commit 37c5a815d8ea33247968212ef4cc83394ceee1bc.

TBR=mtklein

Review URL: https://codereview.chromium.org/635333002
diff --git a/gyp/core.gypi b/gyp/core.gypi
index fb10f32..7c419ed 100644
--- a/gyp/core.gypi
+++ b/gyp/core.gypi
@@ -42,6 +42,7 @@
         '<(skia_src_path)/core/SkBlitter_RGB16.cpp',
         '<(skia_src_path)/core/SkBlitter_Sprite.cpp',
         '<(skia_src_path)/core/SkBuffer.cpp',
+        '<(skia_src_path)/core/SkCachedData.cpp',
         '<(skia_src_path)/core/SkCanvas.cpp',
         '<(skia_src_path)/core/SkChunkAlloc.cpp',
         '<(skia_src_path)/core/SkClipStack.cpp',