Twenty Seventeen: Keep header videos from extending past footer.

This adds 1px of bottom padding to header videos to keep them from
extending past the footer at the bottom of the page.

Fixes #38950.
Built from https://develop.svn.wordpress.org/trunk@39380


git-svn-id: https://core.svn.wordpress.org/trunk@39320 1a063a9b-81f0-0310-95a4-ce76da25c4cd