athena: Introduce ActivityView.

athena/content/app_activity.h

Index: athena/content/app_activity.h
diff --git a/athena/content/app_activity.h b/athena/content/app_activity.h
index db90cfe1eeff03c675ac4f546426e883c00338bb..37ffd208b5c5a06b71019851682e67dfda6d8dfe 100644
--- a/athena/content/app_activity.h
+++ b/athena/content/app_activity.h
@@ -44,6 +44,7 @@ class AppActivity : public Activity,
   SkColor GetRepresentativeColor() const override;
   base::string16 GetTitle() const override;
   gfx::ImageSkia GetIcon() const override;
+  void SetActivityView(ActivityView* activity_view) override;
   bool UsesFrame() const override;
   views::View* GetContentsView() override;
   gfx::ImageSkia GetOverviewModeImage() override;
@@ -86,6 +87,11 @@ class AppActivity : public Activity,
   // The content proxy.
   scoped_ptr<ContentProxy> content_proxy_;
 
+  // WebActivity does not take ownership of |activity_view_|. If the view is
+  // destroyed before the activity, then it must be reset using
+  // SetActivityView(nullptr).
+  ActivityView* activity_view_;
+
   DISALLOW_COPY_AND_ASSIGN(AppActivity);
 };