Adds in-out easing type.

chrome/browser/resources/vr_shell/vr_shell_ui_api.js

Index: chrome/browser/resources/vr_shell/vr_shell_ui_api.js
diff --git a/chrome/browser/resources/vr_shell/vr_shell_ui_api.js b/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
index b3306be198b90fba7dc3cc3b2f3c51c050c7b0d7..48d2f26cba93a84bfa47e0c88664d8208c036dee 100644
--- a/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
+++ b/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
@@ -366,7 +366,8 @@ api.EasingType = {
   'LINEAR': 0,
   'CUBICBEZIER': 1,
   'EASEIN': 2,
-  'EASEOUT': 3
+  'EASEOUT': 3,
+  'EASEINOUT': 4
 };
 
 /** @const */ var DEFAULT_EASING_POW = 2;
@@ -419,6 +420,13 @@ api.OutEasing = class extends api.Easing {
   }
 }
 
+api.InOutEasing = class extends api.Easing {
+  constructor(pow = DEFAULT_EASING_POW) {
+    super(api.EasingType.EASEINOUT);
+    this.pow = pow;
+  }
+}
+
 /**
  * Base animation class. An animation can vary only one object property.
  * @struct