Allow the user to cancel the capture region action, and show its active status in the tray.

ash/common/system/chromeos/palette/palette_ids.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_COMMON_SYSTEM_CHROMEOS_PALETTE_PALETTE_IDS_H_
 #define ASH_COMMON_SYSTEM_CHROMEOS_PALETTE_PALETTE_IDS_H_
 
 #include <string>
 
 #include "ash/ash_export.h"
 
 namespace ash {
 
 // Palette tools are grouped into different categories. Each tool corresponds to
 // exactly one group, and at most one tool can be active per group. Actions are
 // actions the user wants to do, such as take a screenshot, and modes generally
-// change OS behavior, like showing a laser pointer instead of a cursor.
+// change OS behavior, like showing a laser pointer instead of a cursor. A mode
+// is active until the user completes the action or disables it.
 enum class PaletteGroup { ACTION, MODE };
 
 enum class PaletteToolId {
   NONE,
   CREATE_NOTE,
   CAPTURE_REGION,
   CAPTURE_SCREEN,
   LASER_POINTER,
   MAGNIFY,
 };
 
 // Helper functions that convert PaletteToolIds and PaletteGroups to strings.
 ASH_EXPORT std::string PaletteToolIdToString(PaletteToolId tool_id);
 ASH_EXPORT std::string PaletteGroupToString(PaletteGroup group);
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_CHROMEOS_PALETTE_PALETTE_IDS_H_