Render PDFs in <embed>/<object> elements.

Tests:
<object> http://plugindoc.mozdev.org/testpages/pdf.html
<embed> http://acroeng.adobe.com/wp/?page_id=161

This detection uses rate-limited mutation observers to detect <embed>/<object>
elements. This implementation has been profiled and optimized for the common
case, i.e. documents where no <embed> or <object> elements exist.
For most documents, the implementation utilizes a few milliseconds at most.
For huge documents (100k+ nodes), the initial overhead is usually ~5%.

After detecting an <embed>/<object> element, Shadow DOM is applied to render
an iframe instead of the <embed>/<object> element. Chrome 33+ only.

Note: This commit contains code used to profile the performance impact.
Remove code used to profile performance