Adds in-out easing type.

chrome/browser/android/vr_shell/easing.cc

Index: chrome/browser/android/vr_shell/easing.cc
diff --git a/chrome/browser/android/vr_shell/easing.cc b/chrome/browser/android/vr_shell/easing.cc
index 8138c6dd1f51f4e53d6cf1bb671ae35aee870411..137bcb5f66a3eff0d7686401af5b9093acf89244 100644
--- a/chrome/browser/android/vr_shell/easing.cc
+++ b/chrome/browser/android/vr_shell/easing.cc
@@ -33,6 +33,15 @@ double EaseOut::CalculateValueImpl(double state) {
   return 1.0 - pow(1.0 - state, power_);
 }
 
+EaseInOut::EaseInOut(double power) : ease_in_(power) {}
+double EaseInOut::CalculateValueImpl(double state) {
+  if (state < 0.5) {
+    return ease_in_.CalculateValueImpl(state * 2) / 2;
+  } else {
+    return 1.0 - ease_in_.CalculateValueImpl((1.0 - state) * 2) / 2;
+  }
+}
+
 double Linear::CalculateValueImpl(double state) {
   return state;
 }