[CRX] Respect download requests in main frame

[Rob--W]

When the URL contains "=download", respect the Content-Disposition header in the Chrome extension.

Steps to verify:

1. node make chromium
2. Load the extension at chrome://extensions
3. Visit https://drive.google.com/a/notable.ac/file/d/0B6YhVF9p-y2AbUJlcEI5ZGZ5Z2c/view?pli=1
4. Click on the download button.
5. Verify that the PDF is downloaded instead of being opened in the PDF.js Chrome extension.

Fixes half of #6106 (the printing part is still a todo, which I'll address after #6190 gets merged)

[timvandermeij]

Works fine, thank you!