The active handler for a video header isn't accessible

[bradyvercher]

In wp-custom-header.js, the handler used for displaying the video should be assigned to wp.customHeader.activeHandler for other scripts to access. However, with the way it's currently being assigned, that property is always undefined.

For reference, ​this is where the assignment currently happens.

[bradyvercher]

39238.diff​ assigns the handler to the activeHandler properly immediately before calling the handler's initialize() method to ensure the property always has the correct value.