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

chrome/browser/flag_descriptions.cc

Index: chrome/browser/flag_descriptions.cc
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index ce5c3233123dac99fa07a1bc50864b9ab0a6e6f9..4afbbe093702fa590ce2d178a12bb956811dd9e4 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -734,6 +734,12 @@ const char kAshEnableMirroredScreenDescription[] =
     "Enable the mirrored screen mode. This mode flips the screen image "
     "horizontally.";
 
+const char kAshEnableSmoothScreenRotationName[] =
+    "Enable smooth rotation animations.";
+
+const char kAshEnableSmoothScreenRotationDescription[] =
+    "Enable smooth rotation animations.";
+
 const char kMaterialDesignInkDropAnimationFast[] = "Fast";
 
 const char kMaterialDesignInkDropAnimationSlow[] = "Slow";