fix PDF loading local files

[diracdeltas]

muon changes are needed in order to allow extensions to access local files. in the meantime, we can workaround this for pdfjs by copying local pdf files to a disk location to chrome-extension://.

fix #2714
requires brave/pdf.js@6b3081e

test plan:

1. git clone https://github.com/brave/pdf.js and run 'gulp chromium'
2. copy 'build/chromium' to '~/Library/Application\ Support/brave-development/Extensions/jdbefljfgobbmcidnmpjamcbhnbphjnb/1.8.251' or whatever your corrresponding userData directory is
3. start Brave with PDFJS enabled and browse to a local PDF file. it should display correctly and the url in the urlbar should be the file: url.

• 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:

[diracdeltas]

found a much easier way, i'm going to close this