[B2G] UI tweak for multi-resolution support on B2G

See issue #3112
This patch do the following:
 - Update outdated B2G extension to make latest pdf.js workable on B2G.
 - Change viewer.html, viewer.css of B2G extension to support multi-resolution.

For the second one, changing image assets are done by makefile of Gaia, while scaling the UI are done by changing css unit from px to rem, and then set font-size through a shared css called responsive.css outside apps, to scale things on different devices. So things we need to do are mainly just change css units and add images.
No javascript changes are needed for this change.

bigger image assets(next/previous page, zoom in/out) are lacked for now and I'll ask UX to add it later, but I think we may start code review first.

I'm on my first time to send pull request on pdf.js so correct me if I'm doing things wrong, thanks a lot!