Make leak counters thread-safe and turn them on by default for Debug

Make leak counters implemented with SK_DECLARE_INST_COUNT thread-safe.
Enable the leak counting for Debug builds.

Protect the instance counter initialization step (initStep) by
using SkOnce.

Makes SkOnce.h part of the public API, since SkInstCnt is public.

Protect the per-class child list shared variable with a per-class mutex.

Changes the behavior in the way that if the child list has been
"cleaned up", it will still try to create subsequent child lists.

BUG=skia:1219
R=robertphillips@google.com, mtklein@google.com, bungeman@gmail.com, bsalomon@google.com, bungeman@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/99483003

git-svn-id: http://skia.googlecode.com/svn/trunk/include@12635 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed