Bug 1867939, part 4: Implemented the ::target-text CSS Pseudo Element. r=emilio,devtools-reviewers,nchevobbe

This patch implements the `::target-text` pseudo element.
Similarly to the Custom Highlight API, this is done implementing
a new Selection type.

Differential Revision: https://phabricator.services.mozilla.com/D195687

layout/generic/nsTextFrame.cpp

@@ -5676,6 +5676,10 @@ bool nsTextFrame::GetSelectionTextColors(SelectionType aSelectionType,
           aHighlightName, aBackground);
       return hasForeground || hasBackground;
     }
+    case SelectionType::eTargetText: {
+      aTextPaintStyle.GetTargetTextColors(aForeground, aBackground);
+      return true;
+    }
     case SelectionType::eURLSecondary:
       aTextPaintStyle.GetURLSecondaryColor(aForeground);
       *aBackground = NS_RGBA(0, 0, 0, 0);