fix PDFJS loading PDFs in new tabs

[diracdeltas]

fix #8364

also adds LOAD_URL_REQUESTED message handler, which is needed for brave/pdf.js#2

test plan:

1. go to https://letsencrypt.org/repository/
2. right click on any of the links to open in a new tab. it should work.
3. right click on any of the links to open in a new private tab. it should work.

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

[bsclifton]

Changes look and work great 😄 ++