--- a/gfx/layers/ipc/ShadowLayerUtilsX11.h
+++ b/gfx/layers/ipc/ShadowLayerUtilsX11.h
@@ -46,17 +46,17 @@ struct SurfaceDescriptorX11 {
     // SurfaceDescriptorX11.
     return mId == aOther.mId;
   }
 
   already_AddRefed<gfxXlibSurface> OpenForeign() const;
 
   MOZ_INIT_OUTSIDE_CTOR Drawable mId;
   MOZ_INIT_OUTSIDE_CTOR XID mFormat; // either a PictFormat or VisualID
-  gfx::IntSize mSize;
+  MOZ_INIT_OUTSIDE_CTOR gfx::IntSize mSize;
   MOZ_INIT_OUTSIDE_CTOR Drawable mGLXPixmap; // used to prevent multiple bindings to the same GLXPixmap in-process
 };
 
 } // namespace layers
 } // namespace mozilla
 
 namespace IPC {