Extend offline browsing runtime caching to include assets in wp-content that in a plugin/theme directory

[westonruter]

Offline browsing currently enables runtime caching for:

Category	Class	Pattern
Uploaded images	WP_Service_Worker_Uploaded_Image_Caching_Component	image file extensions in wp-content/uploads
Core assets	WP_Service_Worker_Core_Asset_Caching_Component	'^' . preg_quote( trailingslashit( includes_url() ), '/' ) . '.*'
Plugin assets	WP_Service_Worker_Plugin_Asset_Caching_Component	'^' . preg_quote( trailingslashit( plugins_url() ), '/' ) . '.*'
Theme assets	WP_Service_Worker_Theme_Asset_Caching_Component	'^(' . implode( '|', $theme_directory_uri_patterns ) . ').*'

Nevertheless, what is not currently accounted for in runtime caching are assets located in wp-content that aren't inside of uploads, themes, or plugins. Namely, as noted in #667 (comment), a caching plugin like Autoptimize will generate scripts and styles that get stored in wp-content/cache. These should be included in the runtime cache when offline browsing is enabled.