Currently, it's possible to configure URL schemes in such a way that Blink believes the origin is not opaque while url::Origin thinks it is opaque: for example, externalfile: URLs are treated as opaque by url::Origin while blink::SecurityOrigin does not treat them as opaque.

Previously, this was not a huge issue: we would always need to go from Blink -> non-Blink, which would convert it to an opaque url::Origin if needed, then send it over IPC.

However, with Mojo in Blink, we can now send blink::SecurityOrigin over IPC directly. This trips the validation logic: when we deserialize a url.mojom.Origin to a url::Origin, we validate that the opaqueness of the url::Origin matches the opaque bit. This fails when sending an externalfile: URL...