ipc: Make MessageAttachment inherit from RefCountedThreadSafe.

ipc/ipc_message_attachment.h

Index: ipc/ipc_message_attachment.h
diff --git a/ipc/ipc_message_attachment.h b/ipc/ipc_message_attachment.h
index e7553d262cf62c34cc13fdf1aac2f432ec7edf3c..5ed27cae21aeb8fe203f0bf355497bd06c5a8d4b 100644
--- a/ipc/ipc_message_attachment.h
+++ b/ipc/ipc_message_attachment.h
@@ -15,7 +15,7 @@ namespace IPC {
 // Auxiliary data sent with |Message|. This can be a platform file descriptor
 // or a mojo |MessagePipe|. |GetType()| returns the type of the subclass.
 class IPC_EXPORT MessageAttachment
-    : public base::RefCounted<MessageAttachment> {
+    : public base::RefCountedThreadSafe<MessageAttachment> {
  public:
   enum Type {
     TYPE_PLATFORM_FILE,          // The instance is |PlatformFileAttachment|.
@@ -30,7 +30,7 @@ class IPC_EXPORT MessageAttachment
 #endif  // OS_POSIX
 
  protected:
-  friend class base::RefCounted<MessageAttachment>;
+  friend class base::RefCountedThreadSafe<MessageAttachment>;
   MessageAttachment();
   virtual ~MessageAttachment();