Bug 1879579 - Clean up CompareTreePosition and related code. r=smaug

Make TreeOrderedArray support what the form controls stuff needs, and
use it instead of custom nsContentUtils stuff.

Differential Revision: https://phabricator.services.mozilla.com/D201261

dom/html/HTMLFormElement.h

@@ -520,7 +520,8 @@ class HTMLFormElement final : public nsGenericHTMLElement {
   // This is needed to properly clean up the bi-directional references
   // (both weak and strong) between the form and its HTMLImageElements.
 
-  nsTArray<HTMLImageElement*> mImageElements;  // Holds WEAK references
+  // Holds WEAK references
+  TreeOrderedArray<HTMLImageElement*> mImageElements;
 
   // A map from an ID or NAME attribute to the HTMLImageElement(s), this
   // hash holds strong references either to the named HTMLImageElement, or