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

chrome/browser/flag_descriptions.h

Index: chrome/browser/flag_descriptions.h
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 032011196fb6dcb641e83c2a1614a305ce4acb33..d47b4b8f8d46ab8766e800ef19bba830dc83cb9e 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -800,6 +800,12 @@ extern const char kAshEnableMirroredScreenName[];
 // Description for the flag to enable the mirrored screen mode.
 extern const char kAshEnableMirroredScreenDescription[];
 
+// Title for the flag to enable smooth rotation animations.
+extern const char kAshEnableSmoothScreenRotationName[];
+
+// Description for the flag to enable smooth rotation animations.
+extern const char kAshEnableSmoothScreenRotationDescription[];
+
 // Description for the flag that sets material design ink drop animation speed
 // of fast.
 extern const char kMaterialDesignInkDropAnimationFast[];