When `wp_oembed_add_provider()` or `wp_oembed_remove_provider()` is called before the `plugins_loaded` hook has, store the values statically on the `WP_oEmbed` object and add them just-in-time when the object is instantiated. 

This ensures that all plugins have an accurate provider list when `apply_filters( 'oembed_providers', $providers )` is called. 

Props kovshenin.
Fixes #28284.

Built from https://develop.svn.wordpress.org/trunk@28846


git-svn-id: http://core.svn.wordpress.org/trunk@28650 1a063a9b-81f0-0310-95a4-ce76da25c4cd