Bug 1519296 - Part 3: Make PresShell::AddOverrideStyleSheet append its sheet rather than prepend r=emilio

We only ever add a single sheet (EditorOverride.css) there anyway.

Depends on D16282

Differential Revision: https://phabricator.services.mozilla.com/D16283

--HG--
extra : moz-landing-system : lando

layout/base/PresShell.cpp

@@ -8259,7 +8259,7 @@ nsresult PresShell::SetAgentStyleSheets(
 }
 
 nsresult PresShell::AddOverrideStyleSheet(StyleSheet* aSheet) {
-  return mStyleSet->PrependStyleSheet(SheetType::Override, aSheet);
+  return mStyleSet->AppendStyleSheet(SheetType::Override, aSheet);
 }
 
 nsresult PresShell::RemoveOverrideStyleSheet(StyleSheet* aSheet) {