Log VisualEditorDOMRetrieved latency data via EventLogging

Follows I74d396423. This patch binds a handler to 'DOM retrieved' events
emitted by VisualEditor and logs them via EventLogging for graphing.

See https://meta.wikimedia.org/wiki/Schema:VisualEditorDOMRetrieved

Change-Id: I42e4d5ba44f81384db002a9ebb9b9535ea9e9596
Merge "Log VisualEditorDOMRetrieved latency data via EventLogging"