Make SigninViewControllerDelegate unregister itself as modal dialog delegate.

BUG=771142
TEST=crashes go away

Change-Id: I6bb1c0986bc16aba253129c57db08eaefc4cc83a
Reviewed-on: https://chromium-review.googlesource.com/698666Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506576}

chrome/browser/ui/signin_view_controller_delegate.cc

@@ -38,7 +38,11 @@ SigninViewControllerDelegate::SigninViewControllerDelegate(
   web_contents_->SetDelegate(this);
 }
 
-SigninViewControllerDelegate::~SigninViewControllerDelegate() {}
+SigninViewControllerDelegate::~SigninViewControllerDelegate() {
+  web_modal::WebContentsModalDialogManager* manager =
+      web_modal::WebContentsModalDialogManager::FromWebContents(web_contents_);
+  manager->SetDelegate(nullptr);
+}
 
 void SigninViewControllerDelegate::AttachDialogManager() {
   web_modal::WebContentsModalDialogManager::CreateForWebContents(web_contents_);

components/web_modal/web_contents_modal_dialog_manager.cc

@@ -77,9 +77,9 @@ void WebContentsModalDialogManager::WillClose(gfx::NativeWindow dialog) {
 
   bool removed_topmost_dialog = dlg == child_dialogs_.begin();
   child_dialogs_.erase(dlg);
-  if (delegate_ && delegate_->IsWebContentsVisible(web_contents()) &&
-      !child_dialogs_.empty() && removed_topmost_dialog &&
-      !closing_all_dialogs_) {
+  if (!closing_all_dialogs_ &&
+      (!child_dialogs_.empty() && removed_topmost_dialog) &&
+      (delegate_ && delegate_->IsWebContentsVisible(web_contents()))) {
     child_dialogs_.front().manager->Show();
   }