Extract shell methods from ApplicationImpl into a base class, and pass this to Initialize() instead.

mash/screenlock/screenlock.h

Index: mash/screenlock/screenlock.h
diff --git a/mash/screenlock/screenlock.h b/mash/screenlock/screenlock.h
index 597848151879c5e163fd6aa7959f0d72081ffd00..ff6b9bbd8454f3dc9b9ef5fe298b1aae8528c309 100644
--- a/mash/screenlock/screenlock.h
+++ b/mash/screenlock/screenlock.h
@@ -29,12 +29,13 @@ class Screenlock : public mojo::ApplicationDelegate,
 
  private:
   // mojo::ApplicationDelegate:
-  void Initialize(mojo::ApplicationImpl* app) override;
+  void Initialize(mojo::Shell* shell, const std::string& url,
+                  uint32_t id) override;
 
   // mash::shell::mojom::ScreenlockStateListener:
   void ScreenlockStateChanged(bool locked) override;
 
-  mojo::ApplicationImpl* app_;
+  mojo::Shell* shell_;
   mojo::TracingImpl tracing_;
   scoped_ptr<views::AuraInit> aura_init_;
   mojo::WeakBindingSet<mash::shell::mojom::ScreenlockStateListener> bindings_;