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 7bb04d619e16bce5b926769dba596f96dd16a3f5..5d752df09e33a1389cb48869c91504524718ab08 100644
--- a/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
+++ b/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
@@ -337,7 +337,8 @@ api.EasingType = {
   'LINEAR': 0,
   'CUBICBEZIER': 1,
   'EASEIN': 2,
-  'EASEOUT': 3
+  'EASEOUT': 3,
+  'EASEINOUT': 4
 };
 
 /**
@@ -380,6 +381,13 @@ api.OutEasing = class extends api.Easing {
   }
 }
 
+api.InOutEasing = class extends api.Easing {
+  constructor(pow) {
+    super(api.EasingType.EASEINOUT);
+    this.pow = pow;
+  }
+}
+
 /**
  * Base animation class. An animation can vary only one object property.
  * @struct