We'll probably want to do something more accurate in the future with a
custom clang static analysis pass which validates that XPIDL interfaces
have the expected vtable and struct layout, however doing so would be
more involved than the string matching done in this patch.
In addition to checking for extra virtual methods, we'll likely also
want to check for data members on interfaces, and reject them unless the
class is marked as `[builtinclass]` in addition to some other attribute
which we'll need to add to prevent them from being implemented in Rust
(as c++ data members will not be reflected by the rust macro).
There were 2 instances of a comment which contained the word 'virtual'
within a CDATA block. These comments were moved out of the CDATA block
to avoid triggering the error.
Differential Revision: https://phabricator.services.mozilla.com/D151068
