Previously we were inconsistent by generating code for enum definitions
but not generating code for dictionaries. With future changes to the
IDL generator to expose helpers to convert to and from IDL values
this produced circular depdendencies. To solve this problem, also
generate the dictionary definitions in bindings headers.
The Cache and CacheStorage interfaces are quite intertwined. A Cache
object can only be created through CacheStorage, so the Cache interface
cannot be tested on its own. So adding just this stub will allow
implementing the CacheStorage interfaces that create Cache objects. We
can then implement the Cache interface piecemeal.
