ash/mash: Add a mus-client that sets up ash to provide the system ui for mash.

ui/base/user_activity/user_activity_detector.cc

Index: ui/base/user_activity/user_activity_detector.cc
diff --git a/ui/base/user_activity/user_activity_detector.cc b/ui/base/user_activity/user_activity_detector.cc
index bffe4796c9e1981587271c965b3d2eb3546a8d2d..4f04b8623b78604fc473b827a352901cf1e9f460 100644
--- a/ui/base/user_activity/user_activity_detector.cc
+++ b/ui/base/user_activity/user_activity_detector.cc
@@ -53,9 +53,7 @@ UserActivityDetector::UserActivityDetector() {
 
   ui::PlatformEventSource* platform_event_source =
       ui::PlatformEventSource::GetInstance();
-#if defined(OS_CHROMEOS) || defined(OS_LINUX)
-  CHECK(platform_event_source);
-#endif
+  // TODO(sad): Need a PES for mus.
   if (platform_event_source)
     platform_event_source->AddPlatformEventObserver(this);
 }
@@ -63,9 +61,6 @@ UserActivityDetector::UserActivityDetector() {
 UserActivityDetector::~UserActivityDetector() {
   ui::PlatformEventSource* platform_event_source =
       ui::PlatformEventSource::GetInstance();
-#if defined(OS_CHROMEOS) || defined(OS_LINUX)
-  CHECK(platform_event_source);
-#endif
   if (platform_event_source)
     platform_event_source->RemovePlatformEventObserver(this);
   g_instance = nullptr;