Make UserActivityDetector a singleton

ui/wm/core/user_activity_detector.cc

Index: ui/wm/core/user_activity_detector.cc
diff --git a/ui/wm/core/user_activity_detector.cc b/ui/wm/core/user_activity_detector.cc
index 56bf563b3d032f11b34b87a5af295b78f11c4b7e..6b06743bc004cc315eb9de9f6a4c88833fdb09aa 100644
--- a/ui/wm/core/user_activity_detector.cc
+++ b/ui/wm/core/user_activity_detector.cc
@@ -14,6 +14,8 @@ namespace wm {
 
 namespace {
 
+UserActivityDetector* g_instance = nullptr;
+
 // Returns a string describing |event|.
 std::string GetEventDebugString(const ui::Event* event) {
   std::string details = base::StringPrintf(
@@ -44,9 +46,17 @@ const int UserActivityDetector::kNotifyIntervalMs = 200;
 const int UserActivityDetector::kDisplayPowerChangeIgnoreMouseMs = 1000;
 
 UserActivityDetector::UserActivityDetector() {
+  CHECK(!g_instance);
+  g_instance = this;
 }
 
 UserActivityDetector::~UserActivityDetector() {
+  g_instance = nullptr;
+}
+
+// static
+UserActivityDetector* UserActivityDetector::Get() {
+  return g_instance;
 }
 
 bool UserActivityDetector::HasObserver(UserActivityObserver* observer) const {