Unity RectTransformでWebView制御 ref: https://qiita.com/GeneralD/items/34bfbc1e1e90b7725413

ComponentExtensions.cs

using System;
using System.Collections.Generic;
using UnityEngine;

public static class ComponentExtensions {
    public static T GetOrAddComponent<T>(this Component component) where T : Component => component.GetComponent<T>() ?? component.gameObject.AddComponent<T>();
}

WebViewRectAllocator.cs

using System;
using System.Linq;
using UniRx;
using UniRx.Triggers;
using UnityEngine;
using UnityEngine.UI;

[RequireComponent(typeof(RectTransform))]
public class WebViewRectAllocator : MonoBehaviour {

    public StringReactiveProperty URL = new StringReactiveProperty();

    [SerializeField] private Button goBackButton;
    [SerializeField] private Button goForwardButton;

    private void Awake() {
        var webViewObject = this.GetOrAddComponent<WebViewObject>();
        webViewObject.Init(msg => Debug.Log(msg));

        URL.Where(url => !string.IsNullOrEmpty(url))
           .Subscribe(webViewObject.LoadURL);

        GetComponentsInParent<CanvasGroup>()
            .Select(cg => cg.ObserveEveryValueChanged(c => c.alpha >= 1f))
            .CombineLatestValuesAreAllTrue()
            .CombineLatest(this.ObserveEveryValueChanged(self => self.enabled), (l, r) => l && r)
            .Subscribe(webViewObject.SetVisibility)
            .AddTo(this);

        var rect = GetComponent<RectTransform>();
        rect.OnRectTransformDimensionsChangeAsObservable()
            .Merge(Observable.Return(Unit.Default))
            .Select(_ => new Vector3[4])
            .Do(rect.GetWorldCorners)
            .Select(corners => new Vector4(corners.Min(c => c.x), corners.Max(c => c.x), corners.Min(c => c.y), corners.Max(c => c.y)))
            .SelectMany(lrbt => Observable.Return(new Vector3[4])
                        .Do(rect.GetComponentInParent<Canvas>().GetComponent<RectTransform>().GetWorldCorners)
                        .Select(corners => new Vector4(corners.Min(c => c.x), corners.Max(c => c.x), corners.Min(c => c.y), corners.Max(c => c.y)))
                        .Select(canvasLRBT => canvasLRBT - lrbt))
            .Select(lrbt => new { left = (int)-lrbt.x, right = (int)lrbt.y, bottom = (int)-lrbt.z, top = (int)lrbt.w })
            .CatchIgnore()
            .Do(t => Debug.LogFormat("WebView margins: top={0}, left={1}, right={2}, bottom={3}", t.top, t.left, t.right, t.bottom))
            .Subscribe(t => webViewObject.SetMargins(t.left, t.top, t.right, t.bottom))
            .AddTo(this);

        var checkInterval = 2f;

        if (GoBackButton != null) Observable
            .Interval(TimeSpan.FromSeconds(checkInterval))
            .Where(_ => webViewObject.GetVisibility())
            .Select(_ => webViewObject.CanGoBack())
            .BindToButtonOnClick(GoBackButton, _ => webViewObject.GoBack());

        if (GoForwardButton != null) Observable
            .Interval(TimeSpan.FromSeconds(checkInterval))
            .Where(_ => webViewObject.GetVisibility())
            .Select(_ => webViewObject.CanGoForward())
            .BindToButtonOnClick(GoForwardButton, _ => webViewObject.GoForward());
    }

}