Adds in-out easing type.

chrome/browser/android/vr_shell/easing.h

Index: chrome/browser/android/vr_shell/easing.h
diff --git a/chrome/browser/android/vr_shell/easing.h b/chrome/browser/android/vr_shell/easing.h
index c3a025e9a2b8f500c8b4d163f968c0fbfb62d5a4..e4ae5c6be315e84aa7de306a21c8e059c2a6c5a2 100644
--- a/chrome/browser/android/vr_shell/easing.h
+++ b/chrome/browser/android/vr_shell/easing.h
@@ -15,7 +15,8 @@ enum EasingType {
   LINEAR = 0,
   CUBICBEZIER,
   EASEIN,
-  EASEOUT
+  EASEOUT,
+  EASEINOUT,
 };
 
 // Abstract base class for custom interpolators, mapping linear input between
@@ -78,6 +79,18 @@ class EaseOut : public Easing {
   DISALLOW_COPY_AND_ASSIGN(EaseOut);
 };
 
+// Starts with EaseIn and finishes with EaseOut.
+class EaseInOut : public Easing {
+ public:
+  explicit EaseInOut(double power);
+  double CalculateValueImpl(double input) override;
+
+ private:
+  EaseIn ease_in_;
+  EaseOut ease_out_;
+  DISALLOW_COPY_AND_ASSIGN(EaseInOut);
+};
+
 }  // namespace easing
 }  // namespace vr_shell