There are several anti-patterns for the use of AllowJavascript in CrOS Settings WebUI handlers. We should fix these to follow the following guidelines:

1. In general, AllowJavascript should be called by C++ handlers that respond with a JS call (note: not in the response). This makes it clear that JS is allowed at a point when the Web UI is able to respond to the JS call.

2. If JS calls are invoked from observer methods, the WbeUI must make a JS call into the C++ code to indicate that it is ready (in Polymer this is typically in the attached() or ready() method). 
2a. The handler method for the call should call AllowJavascrpt.
2b. The first AllowJavascrpt() call will trigger OnJavascriptallowed() which should set up any observers.