Bug 1909142 part 1: Add Selection and nsFrameSelection getters to enable the accessibility engine to query custom highlight information. r=jjaschke

Differential Revision: https://phabricator.services.mozilla.com/D233805

dom/base/Selection.cpp

@@ -4445,7 +4445,7 @@ void Selection::SetColors(const nsAString& aForegroundColor,
 void Selection::ResetColors() { mCustomColors = nullptr; }
 
 void Selection::SetHighlightSelectionData(
-    HighlightSelectionData aHighlightSelectionData) {
+    struct HighlightSelectionData aHighlightSelectionData) {
   MOZ_ASSERT(mSelectionType == SelectionType::eHighlight);
   mHighlightData = std::move(aHighlightSelectionData);
 }