Multiple inheritance is prohibited by the style guide unless only one superclass has an implementation.  All other superclasses must be pure interfaces with an "Interface" suffix: https://google.github.io/styleguide/cppguide.html#Multiple_Inheritance

Several of the offending classes (see blocked bug for details) are built using multiple inheritance with WebStateObserver and WebStateUserData as the superclasses.  These classes can be combined to eliminate multiple inheritance violations for these classes.