Avoid JS errors after [29057]. This is not how `_.debounce()` works.

See #24716.

Built from https://develop.svn.wordpress.org/trunk@29071


git-svn-id: http://core.svn.wordpress.org/trunk@28857 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid: `hasNext` and `hasPrevious` are functions that must be called. Otherwise they are always truthy properties.

See #24716.

Built from https://develop.svn.wordpress.org/trunk@29072


git-svn-id: http://core.svn.wordpress.org/trunk@28858 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid: make the router work when `?item=` is in the URL on page load. This is still a hack, needs to be event-driven. 
See #24716.

Built from https://develop.svn.wordpress.org/trunk@29073


git-svn-id: http://core.svn.wordpress.org/trunk@28859 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid: add disambiguation between generic frames and routers and the router specifically interacting with the grid's frame.
See #24716.

Built from https://develop.svn.wordpress.org/trunk@29074


git-svn-id: http://core.svn.wordpress.org/trunk@28860 1a063a9b-81f0-0310-95a4-ce76da25c4cd