Issue #1671 - Unprefix ::-moz-selection

This actually keeps both pseudo-elements for now, since the prefixed version is
still used internally, but we need the unprefixed version for web compat.
Note: while unprefixing a non-spec-compliant pseudo here, it's exactly in line
with what other browsers do. Nobody is following the spec here and at least
we'll be doing what everyone else is with our unprefixed version.

layout/generic/nsTextFrame.cpp

@@ -3947,11 +3947,18 @@ nsTextPaintStyle::InitSelectionColorsAndShadow()
   if (selectionElement &&
       selectionStatus == nsISelectionController::SELECTION_ON) {
     RefPtr<nsStyleContext> sc = nullptr;
+    // Probe for both selection and -moz-selection
     sc = mPresContext->StyleSet()->
       ProbePseudoElementStyle(selectionElement,
-                              CSSPseudoElementType::mozSelection,
+                              CSSPseudoElementType::selection,
                               mFrame->StyleContext());
-    // Use -moz-selection pseudo class.
+    if (!sc) {
+      sc = mPresContext->StyleSet()->
+        ProbePseudoElementStyle(selectionElement,
+                                CSSPseudoElementType::mozSelection,
+                                mFrame->StyleContext());
+    }
+    // Use selection pseudo class.
     if (sc) {
       mSelectionBGColor =
         sc->GetVisitedDependentColor(eCSSProperty_background_color);