Revert "bug 887454 Remove useragent override for youtube.com"

This reverts commit 5b5f6d8ecce794323b52daf626cee98dd7b6992e.
Merge pull request #10874 from KevinGrandon/revert_bug_887454

Revert "bug 887454 Remove useragent override for youtube.com" r=fzzzy