[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
Merge pull request #3430 from Rob--W/crx-download-pdf

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