[prefs] When emitting the branch changed signal, check the domain as a prefix
[prefs] When emitting the branch changed signal, check the domain as a prefix

The first parameter passed into nsIPrefBranch2::AddObserver should be
interpreted as a prefix to mark out a subset of prefs contained in the
branch rather than as an exact pref name.

clutter-project/clutter-mozembed,clutter-project/clutter-mozembed,clutter-project/clutter-mozembed