[code_review] Comment about potentially undefined function, which can be easily found on Searchfox

[marco-c]

On https://phabricator.services.mozilla.com/D224761, the tool generated a comment asking to check if waitForPdfJSCanvas was defined, but this function is easily found on Searchfox.
We should debug this case and check questions and answers to figure out what we can improve to avoid it.