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:

Committed: http://code.google.com/p/skia/source/detail?r=13180

[hal.canary, 2014-01-24 19:49:30]

lgtm

[commit-bot: I haz the power, 2014-01-24 19:50:43]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/mtklein@google.com/132803005/160001

[commit-bot: I haz the power, 2014-01-24 19:50:57]

Presubmit check for 132803005-160001 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit ERRORS **
Since the CL is editing public API, you must have an LGTM from one of:
('reed@chromium.org', 'reed@google.com', 'bsalomon@chromium.org',
'bsalomon@google.com')

[reed1, 2014-01-24 21:31:32]

lgtm

[commit-bot: I haz the power, 2014-01-24 21:33:08]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/mtklein@google.com/132803005/160001

[commit-bot: I haz the power, 2014-01-24 21:44:09]

Retried try job too often on Build-Mac10.7-Clang-x86-Release-Trybot for step(s)
BuildTests
http://skiabot-master.pogerlabs.com:10117/buildstatus?builder=Build-Mac10.7-C...

[commit-bot: I haz the power, 2014-01-24 22:19:15]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/mtklein@google.com/132803005/290001

[commit-bot: I haz the power, 2014-01-24 22:38:50]

Change committed as 13180