Annotation adder doesn't appear on Wiley PDFs

Opening this here since we don't yet know if this will require action from our developers.

[A user reported in Zendesk](https://hypothesis.zendesk.com/agent/tickets/7322) that they are unable to annotate any PDFs hosted by Wiley Online publications, even though annotation works just fine for HTML documents. As an example, they sent us this link: https://onlinelibrary.wiley.com/doi/pdf/10.4073/csr.2017.14

## Steps to reproduce
1. Go to https://onlinelibrary.wiley.com/doi/pdf/10.4073/csr.2017.14 and activate Hypothesis with the Bookmarklet or browser extension (any browser)
2. Notice that the Hypothesis sidebar appears
3. Select text on the PDF to annotate

## Expected behavior
The annotation adder appears and you can click the "Annotate" or "Highlight" button

## Actual behavior
Nothing happens

## More info

After some investigation we found it's possible to annotate PDFs if you go to the `src` url provided in the `` below:

`

@dwhly has pinged his contact at Wiley, but understandably many of their people are out of the office for the US Thanksgiving holiday. We plan to follow up with them on Monday.