Bug 1367860 (part 4) - Update CSS fallback font when lang changes (stylo). r=manishearth

MozReview-Commit-ID: AeBHXc0mIVn

layout/style/ServoBindings.cpp

@@ -2397,7 +2397,14 @@ Gecko_nsStyleFont_PrefillDefaultForGeneric(nsStyleFont* aFont,
 {
   const nsFont* defaultFont = ThreadSafeGetDefaultFontHelper(aPresContext, aFont->mLanguage,
                                                              aGenericId);
-   aFont->mFont.fontlist = defaultFont->fontlist;
+  // In case of just the language changing, the parent could have had no generic,
+  // which Gecko just does regular cascading with. Do the same.
+  // This can only happen in the case where the language changed but the family did not
+  if (aGenericId != kGenericFont_NONE) {
+    aFont->mFont.fontlist = defaultFont->fontlist;
+  } else {
+    aFont->mFont.fontlist.SetDefaultFontType(defaultFont->fontlist.GetDefaultFontType());
+  }
 }
 
 void