[CRX] Add a pageAction to omnibox showing PDF URL

In Chromium extensions, the viewer's URL looks like this:
`chrome-extension://oemmndcbldboiebfnladdacbdfmadadm/http://example.com/file.pdf`

Furthermore, the PDF Viewer itself can also add something to the reference fragment:
`chrome-extension://oemmndcbldboiebfnladdacbdfmadadm/http://example.com/file.pdf#page=2`

Consequently, it is difficult to copy a clean URL (e.g. for sharing over mail) without having to tidy-up the URL manually.

This commit solves this issue by adding a button to the omnibox, which shows the clean PDF URL on click:
![pdfjs-pageaction](https://f.cloud.github.com/assets/1365071/1698179/656ed042-5f34-11e3-9e3c-696484c7eec5.png)