Bug 1298676 - part 2 - add an AutoIOInterposerDisable RAII class; r=aklotz

This will ensure the I/O Interposer is re-enabled after temporarily
disabling it.

xpcom/build/IOInterposer.h

@@ -8,6 +8,7 @@
 #define mozilla_IOInterposer_h
 
 #include "mozilla/Attributes.h"
+#include "mozilla/GuardObjects.h"
 #include "mozilla/TimeStamp.h"
 
 namespace mozilla {
@@ -272,6 +273,23 @@ public:
   }
 };
 
+class MOZ_RAII AutoIOInterposerDisable final
+{
+public:
+  explicit AutoIOInterposerDisable(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM)
+  {
+    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+    IOInterposer::Disable();
+  }
+  ~AutoIOInterposerDisable()
+  {
+    IOInterposer::Enable();
+  }
+
+private:
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
 } // namespace mozilla
 
 #endif // mozilla_IOInterposer_h