Serializer enable/disable flags need thread safety.

test/cctest/test-serialize.cc

Index: test/cctest/test-serialize.cc
diff --git a/test/cctest/test-serialize.cc b/test/cctest/test-serialize.cc
index c01a6889c35ef9d355458fdf8e7472e1500aecaf..dd422d5c19bc4dde4c877d5fd4499324de157a6f 100644
--- a/test/cctest/test-serialize.cc
+++ b/test/cctest/test-serialize.cc
@@ -266,7 +266,7 @@ static void Serialize() {
 // Test that the whole heap can be serialized.
 TEST(Serialize) {
   if (!Snapshot::HaveASnapshotToStartFrom()) {
-    Serializer::Enable(CcTest::i_isolate());
+    Serializer::RequestEnable(CcTest::i_isolate());
     v8::V8::Initialize();
     Serialize();
   }
@@ -276,7 +276,7 @@ TEST(Serialize) {
 // Test that heap serialization is non-destructive.
 TEST(SerializeTwice) {
   if (!Snapshot::HaveASnapshotToStartFrom()) {
-    Serializer::Enable(CcTest::i_isolate());
+    Serializer::RequestEnable(CcTest::i_isolate());
     v8::V8::Initialize();
     Serialize();
     Serialize();
@@ -375,7 +375,7 @@ DEPENDENT_TEST(DeserializeFromSecondSerializationAndRunScript2,
 TEST(PartialSerialization) {
   if (!Snapshot::HaveASnapshotToStartFrom()) {
     Isolate* isolate = CcTest::i_isolate();
-    Serializer::Enable(isolate);
+    Serializer::RequestEnable(isolate);
     v8::V8::Initialize();
     v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate);
     Heap* heap = isolate->heap();
@@ -526,7 +526,7 @@ DEPENDENT_TEST(PartialDeserialization, PartialSerialization) {
 TEST(ContextSerialization) {
   if (!Snapshot::HaveASnapshotToStartFrom()) {
     Isolate* isolate = CcTest::i_isolate();
-    Serializer::Enable(isolate);
+    Serializer::RequestEnable(isolate);
     v8::V8::Initialize();
     v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate);
     Heap* heap = isolate->heap();