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

ash/common/palette_delegate.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_PALETTE_DELEGATE_H_
 #define ASH_COMMON_PALETTE_DELEGATE_H_
 
 #include "ash/ash_export.h"
 #include "base/callback.h"
 #include "base/callback_list.h"
@@ skipping 36 matching lines @@
   // event.
   virtual bool ShouldAutoOpenPalette() = 0;
 
   // Returns true if the palette should be displayed. This is the one-shot
   // equivalent to AddPaletteEnableListener.
   virtual bool ShouldShowPalette() = 0;
 
   // Take a screenshot of the entire window.
   virtual void TakeScreenshot() = 0;
 
-  // Take a screenshot of a user-selected region.
-  virtual void TakePartialScreenshot() = 0;
+  // Take a screenshot of a user-selected region. |done| is called when the
+  // partial screenshot session has finished; a screenshot may or may not have
+  // been taken.
+  virtual void TakePartialScreenshot(const base::Closure& done) = 0;
+
+  // Cancels any active partial screenshot session.
+  virtual void CancelPartialScreenshot() = 0;
 
  private:
   DISALLOW_ASSIGN(PaletteDelegate);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_PALETTE_DELEGATE_H_