Uses copy request to flatten the layers to do screen rotation animation.

ash/common/ash_switches.cc

Index: ash/common/ash_switches.cc
diff --git a/ash/common/ash_switches.cc b/ash/common/ash_switches.cc
index 6c259f10a5cc22bebe9aaeb646c2e7c41fa1d1a8..c4932043f7a6eaf325b025f3805269f8233b2921 100644
--- a/ash/common/ash_switches.cc
+++ b/ash/common/ash_switches.cc
@@ -61,6 +61,10 @@ const char kAshEnableTouchView[] = "enable-touchview";
 // Enables mirrored screen.
 const char kAshEnableMirroredScreen[] = "ash-enable-mirrored-screen";
 
+// Enables a smoother animation for screen rotation.
+const char kAshEnableSmoothScreenRotation[] =
+    "ash-enable-smooth-screen-rotation";
+
 // Specifies the estimated time (in milliseconds) from VSYNC event until when
 // visible light can be noticed by the user.
 const char kAshEstimatedPresentationDelay[] =