Track DOM retrieved events

When the editor finishes retrieving the target DOM, fire a 'DOM retrieved'
analytic event with the following properties:

* Response time in millisecond.
* Response size in bytes.
* Whether request was a cache hit.
* Value of X-Parsoid-Performance header (or null if unset).

jQuery.byteLength is now a dependency for ext.visualEditor.mediawiki.

Change-Id: I74d3964238927645e847b6e215991bd6b1ebad59
Merge "Track DOM retrieved events"