Google Drive file picker does not appear after authorization in Safari 12

**Steps to reproduce**

1. Using Safari 12, edit an assignment (for example on [seanh's test course](https://hypothesis.instructure.com/courses/4)) and select Hypothesis as the external tool to use.
2. Click "Use Google Picker" on the document selection form and click through the popup to authorize access to Google Drive

**Expected**: After step 2, the Google Drive file picker should appear
**Actual**: After step 2, the Google Drive file picker does not appear in Safari

**Notes**

- I was not able to reproduce the issue in Chrome 71 or Firefox 64 on the same system.

Start at the assignment document-selection form’s “Use Google Picker” flow and reproduce the authorization sequence in Safari 12, comparing it with Chrome or Firefox. Done means the Google Drive file picker appears after authorization in Safari 12 without regressing the working browsers.

Written by the indexing model from the issue text.