Don't use shadow DOM for rendering <object>/<embed>

[Rob--W]

Multiple shadow roots are not supported any more in Chrome 51+ (https://crbug.com/603448#c6), so this patch changes the way that PDF 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.

Tested as follows:

1. Optional: Check out to pull/7202/head (Refactors dependencies of PDFViewerApplication on external services #7202), because in the current master, the Chrome extension fails at loading any PDFs because of an undefined ChromeCom reference.
2. Visit http://pdfobject.com/static.html
3. Observe that PDF.js is shown in all <embed> and <object> tags.
4. Repeat step 2 and 3 for each of the following cases:
   • Visit chrome://plugins, disable the built-in PDF viewer.
   • Enable the built-in PDF viewer again, and enable click-to-play plugins ("Let me choose when to run plugin content" at https://support.google.com/chrome/answer/142064).
   • (by default the built-in viewer is enabled and set to run automatically)
5. Optional: Try the test cases from <object> / <embed> support in Chrome / Opera #4549.

Verified in Chromium 35.0.1916.15 / 49.0.2623.75 / 50.0.2661.75 / 52.0.2714.0 on Linux.

Fixes #7220

[timvandermeij]

Thank you!