Move ownership of AppSorting from ExtensionPrefs to ExtensionSystem

extensions/browser/mock_extension_system.cc

Index: extensions/browser/mock_extension_system.cc
diff --git a/extensions/browser/mock_extension_system.cc b/extensions/browser/mock_extension_system.cc
index 845ab9ee91993504c00a9cdcf38fd8564ffa16be..bd7c51a190c9670f67f07723936f4b278eeaa96e 100644
--- a/extensions/browser/mock_extension_system.cc
+++ b/extensions/browser/mock_extension_system.cc
@@ -50,6 +50,10 @@ QuotaService* MockExtensionSystem::quota_service() {
   return nullptr;
 }
 
+AppSorting* MockExtensionSystem::app_sorting() {
+  return nullptr;
+}
+
 const OneShotEvent& MockExtensionSystem::ready() const {
   return ready_;
 }