Fixes PDF application switch in Firefox.
Merge pull request #3012 from yurydelendik/pdfjs-switch

Fixes PDF application switch in Firefox.
Get PDF.js to work in Incognito mode.

Without this fix, PDFs are displayed as ERR_ADDRESS_UNREACHABLE
in Incognito mode.
Chrome ext: Show URL in box, frame support, ...

Full list feature changes in this commit:
- Support for iframes
- Switched to content-type detection instead of hard-coding a
  case-sensitive check for the .PDF extension
- The PDF's original URL is visible in the omnibox

Known issues:
- Framed PDFs are not displayed if the frames are placed inside tabs
  whose URL is inaccessible by Chrome. Data-URLs, for instance.
Fix download button for top frames and sub frames

When PDF.js is the top frame, and the PDF URL is identical to
the top URL, download would fail. Fixed by adding a ? or & in these
cases.

When PDF.js is embedded in a frame from a different origin, download
would fail because window.open(url, '_parent') is ignored.
Fixed by using a.click() when available.
a.click() works in Chrome 25, Firefox 19, Opera 12.00 and IE 8.
Safari 5.1 does not support a.click()
Compatible for XHTML documents

Previously, when the XHTML doctype is active, checks would
fail because a <div>'s tag name is "div" instead of "DIV".

Also, document.activeElement does not exists in Chrome, for
XHTML documents
Remove "incognito":"split" because of Chrome bug

Chrome bug #224094 "Extension with incognito:split + webRequest
 always crashes when reloaded while incognito is active" is only visible
 to users with EditIssue permission at http://crbug.com/224094

Initially, the "incognito":"split" permission was added because
the viewer would otherwise not be visible in Incognito mode.
This was caused by the fact that the viewer was displayed directly
on the chrome-extension:// scheme.

By switching to replacing the document instead of redirecting,
"incognito":"split" is not needed any more.
Cache-Control with max-age to avoid downloading 2x
Use a.download if available + documentation
Added Chrome extension's author
Added Chrome extension to README.md
Enable PDF viewer on file:// schemes

Note: PDF viewer is disabled for the file:// + incognito
combination, because it's currently impossible to get the combination
to work.

See https://github.com/mozilla/pdf.js/pull/3017#issuecomment-15693432
Hide XHTML error for PDFs

Hides the flashing <parsererror> attribute for PDFs.
Enable Web Worker again

Previously, Web Workers were not created because of a security
policy which does not respect relaxed privileges.