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

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 7b6ea0f4ebe9259d92943574a4b6f413799abb27..07d68a88a00fb4404e3d051fcb84280a0d5ce327 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2489,6 +2489,14 @@ const FeatureEntry kFeatureEntries[] = {
      FEATURE_VALUE_TYPE(chrome::android::kCustomContextMenu)},
 #endif  // OS_ANDROID
 
+#if defined(USE_ASH)
+    {"ash-enable-smooth-screen-rotation-animation",
+     flag_descriptions::kAshEnableSmoothScreenRotationAnimationName,
+     flag_descriptions::kAshEnableSmoothScreenRotationAnimationDescription,
+     kOsCrOS,
+     SINGLE_VALUE_TYPE(ash::switches::kAshEnableSmoothScreenRotationAnimation)},
+#endif  // defined(USE_ASH)
+
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms.xml. See note in
     // histograms.xml and don't forget to run AboutFlagsHistogramTest unit test.