Cancel partial screenshot UI when lock happens.

ash/shell_observer.h

Index: ash/shell_observer.h
diff --git a/ash/shell_observer.h b/ash/shell_observer.h
index d9852bcdf9285d43051936b3a603a158559a5571..3d4bc0223ab0faaa56e99e0a0adb0f69cf448d6a 100644
--- a/ash/shell_observer.h
+++ b/ash/shell_observer.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include "ash/ash_export.h"
+#include "ash/shell.h"  // for LoginStatus
 
 namespace ash {
 
@@ -15,6 +16,16 @@ class ASH_EXPORT ShellObserver {
   // Invoked after the screen's work area insets changes.
   virtual void OnMonitorWorkAreaInsetsChanged() {}
 
+  // Invoked when the user logs in.
+  virtual void OnLoginStateChanged(Shell::LoginStatus status) {}
+
+  // Invoked when the application is exiting.
+  virtual void OnAppTerminating() {}
+
+  // Invoked when the screen is locked (after the lock window is visible) or
+  // unlocked.
+  virtual void OnLockStateChanged(bool locked) {}
+
  protected:
   virtual ~ShellObserver() {}
 };