Re-introduce the partial magnifier

ash/shell_delegate.h

Index: ash/shell_delegate.h
diff --git a/ash/shell_delegate.h b/ash/shell_delegate.h
index bdd3958045e8d6c58d423c81154b15afbc51262b..6a9dd08bb1c6551ae5aa1223a6e9bf818a14e627 100644
--- a/ash/shell_delegate.h
+++ b/ash/shell_delegate.h
@@ -149,8 +149,14 @@ class ASH_EXPORT ShellDelegate {
   // Returns true if high contrast mode is enabled.
   virtual bool IsHighContrastEnabled() const = 0;
 
-  // Invoked to change the mode of the screen magnifier.
-  virtual void SetMagnifier(MagnifierType type) = 0;
+  // Invoked to enable the screen magnifier.
+  virtual void SetMagnifierEnabled(bool enabled) = 0;
+
+  // Invoked to change the type of the screen magnifier.
+  virtual void SetMagnifierType(MagnifierType type) = 0;
+
+  // Returns if the screen magnifier is enabled or not.
+  virtual bool IsMagnifierEnabled() const = 0;
 
   // Returns the current screen magnifier mode.
   virtual MagnifierType GetMagnifierType() const = 0;