Bug 903873 - Finish porting Web Activities to WebIDL; r=bzbarsky

This patch ports over the ActivityRequestHandler interface.

Unfortunately the original design of the interface specifies that
ActivityRequestHandler.source should be of type ActivityOptions which is
a dictionary.  This is not expressible in WebIDL, so for now we're just
using the type 'object' for that.
Bug 985812 - Make a ActivityRequestHandler.source a pure cached frozen dictionary; r=bzbarsky