Refactor ViewsDelegate singleton

ash/shell/content_client/shell_browser_main_parts.cc

Index: ash/shell/content_client/shell_browser_main_parts.cc
diff --git a/ash/shell/content_client/shell_browser_main_parts.cc b/ash/shell/content_client/shell_browser_main_parts.cc
index a7a7c57d47bfc755d35e09dbb108db0a31e067b5..1845c7629ece1aecd26fb7502689bc5d8833fcd5 100644
--- a/ash/shell/content_client/shell_browser_main_parts.cc
+++ b/ash/shell/content_client/shell_browser_main_parts.cc
@@ -108,8 +108,8 @@ void ShellBrowserMainParts::PreMainMessageLoopRun() {
       false, net_log_.get()));
 
   // A ViewsDelegate is required.
-  if (!views::ViewsDelegate::views_delegate)
-    views::ViewsDelegate::views_delegate = new ShellViewsDelegate;
+  if (!views::ViewsDelegate::GetInstance())
+    views_delegate_.reset(new ShellViewsDelegate);
 
   delegate_ = new ash::shell::ShellDelegateImpl;
   // The global message center state must be initialized absent
@@ -159,6 +159,8 @@ void ShellBrowserMainParts::PostMainMessageLoopRun() {
 
   aura::Env::DeleteInstance();
 
+  views_delegate_.reset();
+
   // The keyboard may have created a WebContents. The WebContents is destroyed
   // with the UI, and it needs the BrowserContext to be alive during its
   // destruction. So destroy all of the UI elements before destroying the