chrome.extension -> chrome.runtime for Ubuntu

Addresses issue #3800, where a bug specific to Ubuntu builds of Chromium
break PDF.js
https://bugs.launchpad.net/ubuntu/+source/chromium-browser/+bug/1232575
Merge pull request #3801 from Rob--W/hotfix/crx-ubuntu

chrome.extension -> chrome.runtime for Ubuntu
chrome.runtime -> chrome.extension when needed

Some Ubuntu users are still stuck at Chromium 25.
Merge pull request #3807 from Rob--W/crx-chromium-25

chrome.runtime -> chrome.extension when needed (support Chromium 20-25)
Add missing jsdocs for PDFJS globals.
Inline setScale in parseScale
Merge pull request #3787 from Snuffleupagus/inline-setScale

Inline setScale in parseScale
Merge pull request #3796 from brendandahl/global-docs

Add missing jsdocs for PDFJS globals.