Serializer enable/disable flags need thread safety.

src/serialize.h

Index: src/serialize.h
diff --git a/src/serialize.h b/src/serialize.h
index 1fc125bcd38188e1540dc532a223a2bea6c061d7..93874e6b74daa1ead577bd6190bf7e199856364c 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -456,6 +456,12 @@ class SerializationAddressMapper {
 
 class CodeAddressMap;
 
+enum SerializationState {
+  SERIALIZER_STATE_DISABLED = 0,
+  SERIALIZER_STATE_ENABLED = 1,
+  SERIALIZER_STATE_PERMANENTLY_DISABLED = 2
+};
+
 // There can be only one serializer per V8 process.
 class Serializer : public SerializerDeserializer {
  public:
@@ -474,9 +480,26 @@ class Serializer : public SerializerDeserializer {
   static void Disable();
 
   // Call this when you have made use of the fact that there is no serialization
-  // going on.
-  static void TooLateToEnableNow() { too_late_to_enable_now_ = true; }
-  static bool enabled() { return serialization_enabled_; }
+  // going on. Returns true if serialization was disabled, and now it's
+  // permanetely disabled.
+  static bool TrySetPermanentlyDisabled() {
+    LockGuard<Mutex> lock_guard(&serialization_mutex_);
+    // Changes are only made under the lock.
+    if (serialization_state_ == SERIALIZER_STATE_DISABLED) {
+      serialization_state_ = SERIALIZER_STATE_PERMANENTLY_DISABLED;
+      return true;
+    }
+    return false;
+  }
+  static bool IsPermanentlyDisabled() {
+    LockGuard<Mutex> lock_guard(&serialization_mutex_);
+    return serialization_state_ ==
+        SERIALIZER_STATE_PERMANENTLY_DISABLED;
+  }
+  static bool enabled() {
+    LockGuard<Mutex> lock_guard(&serialization_mutex_);
+    return serialization_state_ == SERIALIZER_STATE_ENABLED;
+  }
   SerializationAddressMapper* address_mapper() { return &address_mapper_; }
   void PutRoot(int index,
                HeapObject* object,
@@ -574,9 +597,10 @@ class Serializer : public SerializerDeserializer {
   int fullness_[LAST_SPACE + 1];
   SnapshotByteSink* sink_;
   ExternalReferenceEncoder* external_reference_encoder_;
-  static bool serialization_enabled_;
-  // Did we already make use of the fact that serialization was not enabled?
-  static bool too_late_to_enable_now_;
+
+  static Mutex serialization_mutex_;
+  static SerializationState serialization_state_;
+
   SerializationAddressMapper address_mapper_;
   intptr_t root_index_wave_front_;
   void Pad();