Using the built-in Unity's in-editor WebView

TwitterWindow.cs

// Using the built-in Unity's in-editor WebView

using UnityEditor;
using UnityEngine;
using System.Reflection;

// cf. http://qiita.com/kyusyukeigo/items/a7d3940b95c4ec224d12 (in Japanese)
public class TwitterWindow : ScriptableObject
{
    [MenuItem("Window/Twitter")]
    static void OpenTwitter()
    {
        var flags = BindingFlags.Public | BindingFlags.Static | BindingFlags.FlattenHierarchy;
#if UNITY_5_4_OR_NEWER
        var type = (typeof(Editor).Assembly).GetType("UnityEditor.Web.WebViewEditorWindowTabs");
        //var type = Types.GetType("UnityEditor.Web.WebViewEditorWindowTabs", "UnityEditor.dll");
#else
        var type = Types.GetType("UnityEditor.Web.WebViewEditorWindow", "UnityEditor.dll");
#endif
        var methodInfo = type.GetMethod("Create", flags);
        methodInfo = methodInfo.MakeGenericMethod(type);
        methodInfo.Invoke(
            null,
            new object[]
            {
                "Twitter",
                "https://mobile.twitter.com",
                200, 500, 800, 600
            });
    }
}