Multiple shadow roots are not supported any more in Chrome 51+
(https://crbug.com/603448#c6), so this patch changes the way that PDFs
are rendered in `<embed>` / `<object>` tags.
I used shadow roots because their content is not visible from the web
page, so the odds of conflicts were minimal. Now I have to render the
PDF frame directly in the page, which can be observed from the page
(unfortunately).
Now the following happens when an embedded PDF tag is detected:
- `<embed>` tags: The type and src attributes are updated.
- `<object>` tags: The type attribute is changed and the fallback
content is set and displayed.
b45e6a7cc9