[base] Add Decrement and assignment operators to AtomicNumber

src/base/atomic-utils.h

Index: src/base/atomic-utils.h
diff --git a/src/base/atomic-utils.h b/src/base/atomic-utils.h
index 6731a810f18c06f7929c21e2ec9c7f5466bccbe0..e19385dcb15742909517cb208f048d10dfee9ea7 100644
--- a/src/base/atomic-utils.h
+++ b/src/base/atomic-utils.h
@@ -19,12 +19,18 @@ class AtomicNumber {
   AtomicNumber() : value_(0) {}
   explicit AtomicNumber(T initial) : value_(initial) {}
 
-  // Returns the newly set value.
+  // Returns the value after incrementing.
   V8_INLINE T Increment(T increment) {
     return static_cast<T>(base::Barrier_AtomicIncrement(
         &value_, static_cast<base::AtomicWord>(increment)));
   }
 
+  // Returns the value after decrementing.
+  V8_INLINE T Decrement(T decrement) {
+    return static_cast<T>(base::Barrier_AtomicIncrement(
+        &value_, -static_cast<base::AtomicWord>(decrement)));
+  }
+
   V8_INLINE T Value() { return static_cast<T>(base::Acquire_Load(&value_)); }
 
   V8_INLINE void SetValue(T new_value) {
@@ -36,6 +42,9 @@ class AtomicNumber {
     return value;
   }
 
+  V8_INLINE T operator+=(T value) { return Increment(value); }
+  V8_INLINE T operator-=(T value) { return Decrement(value); }
+
  private:
   STATIC_ASSERT(sizeof(T) <= sizeof(base::AtomicWord));