Bug 1223193: P1. Make ImageBridgeChild::mShuttingDown atomic. r=nical

gfx/layers/ipc/ImageBridgeChild.h

@@ -9,6 +9,7 @@
 #include <stddef.h>                     // for size_t
 #include <stdint.h>                     // for uint32_t, uint64_t
 #include "mozilla/Attributes.h"         // for override
+#include "mozilla/Atomics.h"
 #include "mozilla/RefPtr.h"             // for already_AddRefed
 #include "mozilla/ipc/SharedMemory.h"   // for SharedMemory, etc
 #include "mozilla/layers/AsyncTransactionTracker.h" // for AsyncTransactionTrackerHolder
@@ -316,7 +317,7 @@ protected:
                                   bool aUnsafe);
 
   CompositableTransaction* mTxn;
-  bool mShuttingDown;
+  Atomic<bool> mShuttingDown;
 };
 
 } // namespace layers