Fix race condition when locking/unlocking pixels.

src/core/SkBitmap.cpp

Index: src/core/SkBitmap.cpp
diff --git a/src/core/SkBitmap.cpp b/src/core/SkBitmap.cpp
index 2635e130666885e1b98ad35e99ac5ba85922d080..9a116dfc2ed511a90f35bb63f35377fdb824fd7b 100644
--- a/src/core/SkBitmap.cpp
+++ b/src/core/SkBitmap.cpp
@@ -340,7 +340,7 @@ SkPixelRef* SkBitmap::setPixelRef(SkPixelRef* pr, size_t offset) {
 }
 
 void SkBitmap::lockPixels() const {
-    if (NULL != fPixelRef && 1 == ++fPixelLockCount) {
+    if (NULL != fPixelRef && 0 == sk_atomic_inc(&fPixelLockCount)) {
         fPixelRef->lockPixels();
         this->updatePixelsFromRef();
     }
@@ -350,7 +350,7 @@ void SkBitmap::lockPixels() const {
 void SkBitmap::unlockPixels() const {
     SkASSERT(NULL == fPixelRef || fPixelLockCount > 0);
 
-    if (NULL != fPixelRef && 0 == --fPixelLockCount) {
+    if (NULL != fPixelRef && 1 == sk_atomic_dec(&fPixelLockCount)) {
         fPixelRef->unlockPixels();
         this->updatePixelsFromRef();
     }