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 91a6bea3d7ce3be482a1ab043729f93ebc2cdac2..ce6fd4237a5e2d2f930c02fd0d17ffc2fb754a1b 100644
--- a/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
+++ b/chrome/browser/resources/vr_shell/vr_shell_ui_api.js
@@ -341,7 +341,8 @@ api.EasingType = {
   'LINEAR': 0,
   'CUBICBEZIER': 1,
   'EASEIN': 2,
-  'EASEOUT': 3
+  'EASEOUT': 3,
+  'EASEINOUT': 4
 };
 
 /** @const */ var DEFAULT_EASING_POW = 2;
@@ -394,6 +395,13 @@ api.OutEasing = class extends api.Easing {
   }
 }
 
+api.InOutEasing = class extends api.Easing {
+  constructor(pow = DEFAULT_EASING_POW) {

[cjgrant, 2017/02/16 20:54:07]

nit: On POW vs POWER:

https://engdoc.corp.google.com/eng/doc/devguide/cpp/styleguide.shtml?cl=head#...

Probably not worth changing this here; just an FYI for code you add later.

+    super(api.EasingType.EASEINOUT);
+    this.pow = pow;
+  }
+}
+
 /**
  * Base animation class. An animation can vary only one object property.
  * @struct