athena: Remove ActivityViewModel::CreateWidget().

athena/content/web_activity.cc

Index: athena/content/web_activity.cc
diff --git a/athena/content/web_activity.cc b/athena/content/web_activity.cc
index b29deef77d7a6eedf6b8e491aa0f7fad374013ad..2aae6e86b5f5a80e419906a4af7878e450a88aa1 100644
--- a/athena/content/web_activity.cc
+++ b/athena/content/web_activity.cc
@@ -528,7 +528,8 @@ Activity::ActivityMediaState WebActivity::GetMediaState() {
 }
 
 aura::Window* WebActivity::GetWindow() {
-  return web_view_->GetWidget()->GetNativeWindow();
+  return web_view_->GetWidget() ? web_view_->GetWidget()->GetNativeWindow()
+                                : nullptr;
 }
 
 content::WebContents* WebActivity::GetWebContents() {
@@ -564,10 +565,6 @@ views::View* WebActivity::GetContentsView() {
   return web_view_;
 }
 
-views::Widget* WebActivity::CreateWidget() {
-  return nullptr;  // Use default widget.
-}
-
 gfx::ImageSkia WebActivity::GetOverviewModeImage() {
   if (content_proxy_.get())
     return content_proxy_->GetContentImage();