Add SkMaskCache

BUG=skia:

Review URL: https://codereview.chromium.org/670063004
diff --git a/gyp/core.gypi b/gyp/core.gypi
index 4f9379f..553e36d 100644
--- a/gyp/core.gypi
+++ b/gyp/core.gypi
@@ -104,6 +104,7 @@
         '<(skia_src_path)/core/SkLineClipper.cpp',
         '<(skia_src_path)/core/SkMallocPixelRef.cpp',
         '<(skia_src_path)/core/SkMask.cpp',
+        '<(skia_src_path)/core/SkMaskCache.cpp',
         '<(skia_src_path)/core/SkMaskFilter.cpp',
         '<(skia_src_path)/core/SkMaskGamma.cpp',
         '<(skia_src_path)/core/SkMaskGamma.h',