mash: Support accessing chrome://settings

Currently, the settings button in the status area does nothing.
Also, navigating to chrome://settings causes a crash:

[1098:1098:0913/131833:FATAL:shell.cc(193)] Check failed: instance_. 
#0 0x7f04505d466e base::debug::StackTrace::StackTrace()
#1 0x7f04506373bc logging::LogMessage::~LogMessage()
#2 0x7f044315b830 ash::Shell::GetInstance()
#3 0x7f04561d8759 chromeos::options::DisplayOptionsHandler::DisplayOptionsHandler()
#4 0x7f0455e6e77a options::OptionsUI::OptionsUI()
#5 0x7f0455ceddff (anonymous namespace)::NewWebUI<>()
#6 0x7f0455cec892 ChromeWebUIControllerFactory::CreateWebUIControllerForURL()
#7 0x7f044b6edf37 content::WebUIControllerFactoryRegistry::CreateWebUIControllerForURL()
#8 0x7f044b68ff6e content::WebContentsImpl::CreateWebUI()
#9 0x7f044b6a1608 content::WebContentsImpl::CreateWebUIForRenderFrameHost()
#10 0x7f044b6a1666 content::WebContentsImpl::CreateWebUIForRenderFrameHost()
#11 0x7f044aebea1f content::RenderFrameHostImpl::UpdatePendingWebUI()
#12 0x7f044aee8bb1 content::RenderFrameHostManager::UpdateStateForNavigate()
#13 0x7f044aee7eed content::RenderFrameHostManager::Navigate()
#14 0x7f044aea9cbc content::NavigatorImpl::NavigateToEntry()