Use atomic operations to increment/decrement memory usage counts as multiple threads could be alloc…

runtime/vm/atomic_macos.h

Index: runtime/vm/atomic_macos.h
diff --git a/runtime/vm/atomic_macos.h b/runtime/vm/atomic_macos.h
index 31d219b027457db06b6d6687a1850c58d07d27a8..1cf951e838aba15ca1471c2af8bd1b4d67109fd2 100644
--- a/runtime/vm/atomic_macos.h
+++ b/runtime/vm/atomic_macos.h
@@ -21,11 +21,23 @@ inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
 }
 
 
+inline uintptr_t AtomicOperations::FetchAndIncrementBy(intptr_t* p,
+                                                       intptr_t value) {
+  return __sync_fetch_and_add(p, value);
+}
+
+
 inline uintptr_t AtomicOperations::FetchAndDecrement(uintptr_t* p) {
   return __sync_fetch_and_sub(p, 1);
 }
 
 
+inline uintptr_t AtomicOperations::FetchAndDecrementBy(intptr_t* p,
+                                                       intptr_t value) {
+  return __sync_fetch_and_sub(p, value);
+}
+
+
 #if !defined(USING_SIMULATOR)
 inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
                                                   uword old_value,