The only thing it's used for is calling will paint on a document tree. To iterate every viewmanager looking for those in one document tree is a bit silly, we can just walk the view tree. Since there are only 3 types of views (root views, popup views for menupopupframes, and subdocument frame views (and their inner view)) we don't have to walk many pointers: menupopupframes are controlled by chrome code and are limited, and the other two types of views correspond to the number of view managers in the tree (one view manager per document).

Differential Revision: https://phabricator.services.mozilla.com/D180677