We can easily create anonymous surveys with Excel Services in Office 365 as described in this post.
However if you want the “Survey” button to be available in Office Web App as described below, make sure the Sharing option “Allow both external users who accepted sharing invitations and anonymous guest links” has been selected at the site collection level. Also, make sure your customer fully understands the consequences of this option: any document can be shared anonymously. Sharing is easy, but managing the shares is more complicated –>this requires more governance and many customers are reluctant to it. A dedicated anonymous site collection might be an option.
Set the following option at the site collection level:
The Survey button shows-up in Excel Web App :