Don't call to a possibly-null manager.

The SigninViewControllerDelegate, as a modal dialog manager
delegate, will outlive the manager. If so, don't call through
to a non-existent manager.

BUG=771872,771142
TBR=wittman@chromium.org

Change-Id: Ia020c026094c3092e4946155d3437c2855951595
Reviewed-on: https://chromium-review.googlesource.com/702515Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506794}

chrome/browser/ui/signin_view_controller_delegate.cc

@@ -41,7 +41,8 @@ SigninViewControllerDelegate::SigninViewControllerDelegate(
 SigninViewControllerDelegate::~SigninViewControllerDelegate() {
   web_modal::WebContentsModalDialogManager* manager =
       web_modal::WebContentsModalDialogManager::FromWebContents(web_contents_);
-  manager->SetDelegate(nullptr);
+  if (manager)
+    manager->SetDelegate(nullptr);
 }
 
 void SigninViewControllerDelegate::AttachDialogManager() {