SkBitmap: Fix race condition under multi-thread environment.

++fPixelLockCount and --fPixelLockCount in lockPixels() and unlockPixels()
is not thread safe. Some race codidions may happened that lead to black
screen in some games.

Change-Id: I22f70467d5fcc8675ef400e5066302c89ff11162
Author: Liang Wang <liangx.wang@intel.com>
Signed-off-by: Liang Wang <liangx.wang@intel.com>
Signed-off-by: Yihua Jin <yihua.jin@intel.com>
Signed-off-by: Bing Deng <bingx.deng@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 37550