Lots of malformed WebIDL files

[richard-uk1]

A lot of the WebIDL files are malformed in the same way: they have an interface declaration like so:

interface nameofinterface;

which is not valid WebIDL. The valid equivalent is

interface nameofinterface {};

We could either change all the webidl files to be correct, or adapt the parser to handle the above form.

[ghost]

The conclusion in #446 is:

it is fine to just comment it out and leave a comment about why it is commented out.

[alexcrichton]

Indeed! We should update the documentation for web-sys to reflect this as well!