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I usually do that in the reverse proxy I have before the server.
Other ways would be to adapt web.xml an map the url to some file or to add a filter to Liferay. You probably can also add a context path to the server.xml, but I never tried that.
And I think, google also allows you other verification methods, e.g. adding a meta tag to the page header. Could also be an option.

You could also upload the verification file to the documents and media library. After that, create a (hidden) page of type "URL" with the desired path (e.g. /google12345.html) which redirects to the download URL of the file in documents and media.
That's we way we do it typically in our Liferay Installations.