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

mash/shelf/shelf_view.cc

Index: mash/shelf/shelf_view.cc
diff --git a/mash/shelf/shelf_view.cc b/mash/shelf/shelf_view.cc
index bf75e48ac21204856e4832963d1a5402ebf6c745..c31500a6b2547bada593819869072f064a233d0a 100644
--- a/mash/shelf/shelf_view.cc
+++ b/mash/shelf/shelf_view.cc
@@ -230,9 +230,9 @@ class ShelfView::StartFadeAnimationDelegate : public gfx::AnimationDelegate {
   DISALLOW_COPY_AND_ASSIGN(StartFadeAnimationDelegate);
 };
 
-ShelfView::ShelfView(mojo::ApplicationImpl* app)
-    : app_(app),
-      model_(app),
+ShelfView::ShelfView(mojo::Shell* shell)
+    : shell_(shell),
+      model_(shell),
       alignment_(SHELF_ALIGNMENT_BOTTOM),
       first_visible_index_(0),
       last_visible_index_(-1),