allow PDF.js to access canvas (without warning dialog)
As noted by Mike Perry in a comment to ticket:7265#comment:6, the "site is trying to access canvas" warning is sometimes displayed when viewing a PDF file in-browser using PDF.js.
As the warning is a false positive here (PDF.js can safely access the canvas without putting the user at the risk of fingerprinting), access to the canvas should be allowed without any prompt or warning.