Ship the PDF.js extension with our Chrome extension

This is what this does:

* Take our PDF.js fork (from https://github.com/csillag/pdf.js/, for-h branch)
* Build the Chrome extension from it
* Add the contents of the plugin to h/browser/chrome_pdfjs/
* Merge the contents of manifest.json from both extensions
* Modify the extension building script (h/script.py) to include these files in the extension.

  * * *

Rebased to latest develop (which have the PDF driver code), and all known PDF-related bugs were resolved.

(For a demo of how this looks when built, see the [extension in the Chrome web store](https://chrome.google.com/webstore/detail/pdf%20hypothesis/bipacimpfefoidapjkknffflfpfmjdog), which was built using this code.).

This PR obsoletes #894.