Customize: Avoid unnecessary `get_theme_mod()` call and premature escaping in...

Customize: Avoid unnecessary `get_theme_mod()` call and premature escaping in `get_header_video_url()`.

The result is still escaped with `esc_url_raw()` for retrieval, and with `esc_url()` for display in `the_header_video_url()`.

Props dlh, Tkama.
Fixes #40044.
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git-svn-id: https://core.svn.wordpress.org/trunk@47067 1a063a9b-81f0-0310-95a4-ce76da25c4cd