[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.
Merge pull request #4006 from Rob--W/crx-url-in-pageaction

[CRX] Add a pageAction to omnibox showing PDF URL