Avoid data races on initializing GCScope.

third_party/WebKit/Source/platform/heap/SafePoint.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SafePoint_h
 #define SafePoint_h
 
 #include "platform/heap/ThreadState.h"
 #include "wtf/ThreadingPrimitives.h"
 
 namespace blink {
 
 class SafePointScope final {
     WTF_MAKE_NONCOPYABLE(SafePointScope);
 public:
-    explicit SafePointScope(BlinkGC::StackState stackState, ThreadState* state = ThreadState::current())
+    explicit SafePointScope(BlinkGC::StackState stackState, ThreadState* state = ThreadState::current(), bool delayEnter = false)
         : m_state(state)
     {
-        if (m_state) {
-            RELEASE_ASSERT(!m_state->isAtSafePoint());
-            m_state->enterSafePoint(stackState, this);
-        }
+        if (delayEnter)
+            return;
+        enter(stackState);
     }
 
     ~SafePointScope()
     {
         if (m_state)
             m_state->leaveSafePoint();
     }
 
+    void enter(BlinkGC::StackState stackState)
+    {
+        if (!m_state)
+            return;
+
+        RELEASE_ASSERT(!m_state->isAtSafePoint());
+        m_state->enterSafePoint(stackState, this);
+    }
+
 private:
     ThreadState* m_state;
 };
 
 // The SafePointAwareMutexLocker is used to enter a safepoint while waiting for
 // a mutex lock. It also ensures that the lock is not held while waiting for a GC
 // to complete in the leaveSafePoint method, by releasing the lock if the
 // leaveSafePoint method cannot complete without blocking, see
 // SafePointBarrier::checkAndPark.
 class SafePointAwareMutexLocker final {
@@ skipping 82 matching lines @@
     volatile int m_canResume;
     volatile int m_unparkedThreadCount;
     Mutex m_mutex;
     ThreadCondition m_parked;
     ThreadCondition m_resume;
 };
 
 } // namespace blink
 
 #endif