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.
Define MediaKeySystemConfiguration and MediaKeySystemMediaCapability in
Web::EncryptedMediaExtensions instead of Web::Bindings. This matches
other dictionary definitions in the codebase.
