Don't pass $this by reference to SkinVectorStyleModules hook.

Fix for Ica0b69ad95 (8b6ed4d0).

$this cannot be changed, so there is not any reason to pass
it by reference.

Change-Id: I916ccabbaff9f60edb344550111f62c833bb137b
Merge "Don't pass $this by reference to SkinVectorStyleModules hook."