diff --git a/content/browser/WebPageImpl.cpp b/content/browser/WebPageImpl.cpp
index ba8cb9c606..01709509ca 100644
--- a/content/browser/WebPageImpl.cpp
+++ b/content/browser/WebPageImpl.cpp
@@ -89,6 +89,7 @@ void WebPageImpl::initBlink()
 
 void WebPageImpl::registerDestroyNotif(DestroyNotif* destroyNotif)
 {
+    WTF::Locker<WTF::Mutex> locker(m_destroyNotifsMutex);
     size_t pos = m_destroyNotifs.find(destroyNotif);
     if (WTF::kNotFound == pos)
         m_destroyNotifs.append(destroyNotif);
@@ -96,6 +97,8 @@ void WebPageImpl::registerDestroyNotif(DestroyNotif* destroyNotif)
 
 void WebPageImpl::unregisterDestroyNotif(DestroyNotif* destroyNotif)
 {
+    RELEASE_ASSERT(WTF::isMainThread());
+    WTF::Locker<WTF::Mutex> locker(m_destroyNotifsMutex);
     size_t pos = m_destroyNotifs.find(destroyNotif);
     if (WTF::kNotFound != pos)
         m_destroyNotifs.remove(pos);
diff --git a/content/browser/WebPageImpl.h b/content/browser/WebPageImpl.h
index e70bf34d1e..a48d295258 100644
--- a/content/browser/WebPageImpl.h
+++ b/content/browser/WebPageImpl.h
@@ -290,6 +290,7 @@ class WebPageImpl
     bool m_postCloseWidgetSoonMessage;
 
     WTF::Vector<DestroyNotif*> m_destroyNotifs;
+    WTF::Mutex m_destroyNotifsMutex;
     HRGN m_draggableRegion;
 
     HWND m_popupHandle;