Fix inline settings toggle crash.

ToggleInlineSettings assumed |settings_row_| to exist. However, it
wasn't always the case.

TEST=manual
BUG=829240

Change-Id: I0a95bf830f616fd7c380fdf19009da00f53ba3b3
Reviewed-on: https://chromium-review.googlesource.com/1053933Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557941}

ui/message_center/views/notification_view_md.cc

@@ -1232,7 +1232,8 @@ void NotificationViewMD::UpdateViewForExpandedState(bool expanded) {
 }
 
 void NotificationViewMD::ToggleInlineSettings(const ui::Event& event) {
-  DCHECK(settings_row_);
+  if (!settings_row_)
+    return;
 
   bool inline_settings_visible = !settings_row_->visible();