[Chrome extension] Added JSDocs to pdfHandler.js

And a small refactor to reduce the LOC in the onHeadersReceived
callback, to improve the readability of the code.
[CRX] Don't intercept download of PDF attachment

Fixes #3426
Syncing order of JS files in viewer.html and make.js. See #3414
Removes foreign for Firefox CSS prefixes
Merge pull request #3457 from yurydelendik/remove-prefixes

Removes foreign for Firefox CSS prefixes
Merge pull request #3440 from peerlibrary/syncing-order

Syncing order of JS files in viewer.html and make.js. See #3414
Merge pull request #3430 from Rob--W/crx-download-pdf

Chrome extension: Do not intercept PDF downloads if it's target is a frame