Bug 1780878: [Part 2] Cache DESCRIBED_BY and DESCRIPTION_FOR relations r=Jamie

Differential Revision: https://phabricator.services.mozilla.com/D152580

accessible/base/CacheConstants.h

@@ -73,6 +73,8 @@ static constexpr RelationData kRelationTypeAtoms[] = {
      RelationType::CONTROLLED_BY},
     {nsGkAtoms::_for, nsGkAtoms::output, RelationType::CONTROLLED_BY,
      RelationType::CONTROLLER_FOR},
+    {nsGkAtoms::aria_describedby, nullptr, RelationType::DESCRIBED_BY,
+     RelationType::DESCRIPTION_FOR},
 };
 
 }  // namespace a11y

accessible/generic/LocalAccessible.cpp

@@ -1334,6 +1334,7 @@ void LocalAccessible::DOMAttributeChanged(int32_t aNameSpaceID,
   }
 
   if (aAttribute == nsGkAtoms::aria_describedby) {
+    mDoc->QueueCacheUpdate(this, CacheDomain::Relations);
     mDoc->FireDelayedEvent(nsIAccessibleEvent::EVENT_DESCRIPTION_CHANGE, this);
     if (aModType == dom::MutationEvent_Binding::MODIFICATION ||
         aModType == dom::MutationEvent_Binding::ADDITION) {