Magnifier: Prevent useless operation in enabling/disabling magnifier.

ash/magnifier/magnification_controller.cc

Index: ash/magnifier/magnification_controller.cc
diff --git a/ash/magnifier/magnification_controller.cc b/ash/magnifier/magnification_controller.cc
index d2169b320e0a88530ee0baf6b7399ea91fe48224..1cbc56a2a60b0e5ca2079c0f3e24f342007e8a0d 100644
--- a/ash/magnifier/magnification_controller.cc
+++ b/ash/magnifier/magnification_controller.cc
@@ -472,6 +472,9 @@ void MagnificationControllerImpl::EnsurePointIsVisible(
 }
 
 void MagnificationControllerImpl::SetEnabled(bool enabled) {
+  if (enabled == is_enabled_)
+    return;
+
   if (enabled) {
     float scale =
         ash::Shell::GetInstance()->delegate()->GetSavedScreenMagnifierScale();