SkOnce: add option to call another cleanup function once at exit.

SkOnce: add option to call another cleanup function once at exit.

Use this to clean up empty SkData and SkPathRef.

Current leaks:
  Leaked SkRefCntBase: 40
     Leaked SkFlattenable: 32
         Leaked SkPixelRef: 32
             Leaked SkMallocPixelRef: 32
     Leaked SkFontConfigInterface: 1
     Leaked SkWeakRefCnt: 1
         Leaked SkTypeface: 1
     Leaked SkFontMgr: 1
     Leaked SkDataTable: 3
     Leaked SkImage: 1
         Leaked ???: 1
     Leaked ???: 1

BUG=skia:
R=halcanary@google.com, reed@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/132803005

git-svn-id: http://skia.googlecode.com/svn/trunk@13180 2bbb7eff-a529-9590-31e7-b0007b416f81