Created attachment 461825 [details]
Test Case

The assertion `console.assert(didDelete, this, eventType, listener, thisObject);` is being hit in `Base/Object.js removeEventListener` when loading the attached test case. (Alternate WebM Player feature is enabled).

Object logged:
```
DOMNodeStyles {_listeners: Multimap, _node: DOMNode, _rulesMap: Map, _stylesMap: Multimap, _matchedRules: [], …}
"dom-node-styles-needs-refresh"
function _handleNodesStylesNeedsRefresh(event) { this._needsVariablesFromInspectedNode = true; }
CSSPropertyNameCompletions {_values: Array, _acceptEmptyPrefix: false, _queryController: null, _cachedSortedPropertyNames: Array, _needsVariablesFromInspectedNode: true}
```