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

ash/common/palette_delegate.h

Index: ash/common/palette_delegate.h
diff --git a/ash/common/palette_delegate.h b/ash/common/palette_delegate.h
index c84bc00cb1226828694968214e63020337d39121..3aa81697d32e46aeeccea218252c228277210c2b 100644
--- a/ash/common/palette_delegate.h
+++ b/ash/common/palette_delegate.h
@@ -54,8 +54,13 @@ class PaletteDelegate {
   // 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);