diff -r 000000000000 -r 6474c204b198 dom/camera/AutoRwLock.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/camera/AutoRwLock.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,49 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef RWLOCK_AUTO_ENTER_H
+#define RWLOCK_AUTO_ENTER_H
+
+#include "prrwlock.h"
+#include "mozilla/Assertions.h"
+
+class RwLockAutoEnterRead
+{
+public:
+  RwLockAutoEnterRead(PRRWLock* aRwLock)
+    : mRwLock(aRwLock)
+  {
+    MOZ_ASSERT(mRwLock);
+    PR_RWLock_Rlock(mRwLock);
+  }
+
+  ~RwLockAutoEnterRead()
+  {
+    PR_RWLock_Unlock(mRwLock);
+  }
+
+protected:
+  PRRWLock* mRwLock;
+};
+
+class RwLockAutoEnterWrite
+{
+public:
+  RwLockAutoEnterWrite(PRRWLock* aRwLock)
+    : mRwLock(aRwLock)
+  {
+    MOZ_ASSERT(mRwLock);
+    PR_RWLock_Wlock(mRwLock);
+  }
+
+  ~RwLockAutoEnterWrite()
+  {
+    PR_RWLock_Unlock(mRwLock);
+  }
+
+protected:
+  PRRWLock* mRwLock;
+};
+
+#endif // RWLOCK_AUTO_ENTER_H