Convert ==/!= to ===/!== in: //chrome/browser/resources/pdf and tests

CL generated following the example of
https://chromium-review.googlesource.com/c/chromium/src/+/2003795
by dbeam@

$ git ls-files 'chrome/browser/resources/pdf/*.js' | xargs grep -l -P ' [!=]= (?!null)' | xargs perl -p -i -e 's/ ([!=])= (?!null)/ \1== /g'

$ git ls-files 'chrome/test/data/pdf/*.js' | xargs grep -l -P ' [!=]= (?!null)' | xargs perl -p -i -e 's/ ([!=])= (?!null)/ \1== /g'

Also updating one location where a number was being compared to a
string, and adding eslint rules to prevent any new ==/!= from being
added.

Bug: 720034
Change-Id: I0b172e297c9dd873ee2228233a26224332983e6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2064789
Reviewed-by: Demetrios Papadopoulos <[email protected]>
Commit-Queue: Rebekah Potter <[email protected]>
Cr-Commit-Position: refs/heads/master@{#742907}
28 files changed