athena: Remove ActivityViewModel::CreateWidget().

athena/content/app_activity_proxy.cc

Index: athena/content/app_activity_proxy.cc
diff --git a/athena/content/app_activity_proxy.cc b/athena/content/app_activity_proxy.cc
index c2d0e34de0ef8335ceb5c3bee3150cc27c2fdeea..77953ced19921c51c87baea81eca70fa0feff2d2 100644
--- a/athena/content/app_activity_proxy.cc
+++ b/athena/content/app_activity_proxy.cc
@@ -52,7 +52,7 @@ Activity::ActivityMediaState AppActivityProxy::GetMediaState() {
 }
 
 aura::Window* AppActivityProxy::GetWindow() {
-  return view_->GetWidget()->GetNativeWindow();
+  return view_->GetWidget() ? view_->GetWidget()->GetNativeWindow() : nullptr;
 }
 
 content::WebContents* AppActivityProxy::GetWebContents() {
@@ -92,10 +92,6 @@ views::View* AppActivityProxy::GetContentsView() {
   return view_;
 }
 
-views::Widget* AppActivityProxy::CreateWidget() {
-  return nullptr;
-}
-
 gfx::ImageSkia AppActivityProxy::GetOverviewModeImage() {
   return content_proxy_->GetContentImage();
 }