Customize: Use selective refresh to preview changes to site title and tagline in core themes.

Fixes issue where `wptexturize` and other filters fail to apply when previewing changes via `postMessage` transport.

See #27355.
Fixes #33738.

Built from https://develop.svn.wordpress.org/trunk@36797


git-svn-id: https://core.svn.wordpress.org/trunk@36764 1a063a9b-81f0-0310-95a4-ce76da25c4cd