#環境
Unity2017.1.0f3

#概要
RectTransformの矩形からスクリーン矩形に変換します
主にWebViewの表示領域に使用します

コガネブログさんが作成したのを使用すると良いと思います。
http://baba-s.hatenablog.com/entry/2019/06/21/090000

#GreeのWebView
GreeのWebViewをWindowsのUnityEditorである程度動作させるとっかかり

#プログラム

using UnityEngine;

public class TestRectTransformToScreenSpaceRect : MonoBehaviour
{
	[SerializeField] RectTransform _rectTrans = null;

	private Rect _screenRect;

	private void Update ()
	{
		_screenRect = CalcurateScreenSpaceRect( _rectTrans );
	}

	private static Rect CalcurateScreenSpaceRect( RectTransform rectTrans )
	{
		var canvas = rectTrans.GetComponentInParent<Canvas>();
		var camera = canvas.worldCamera;
		var corners = new Vector3[ 4 ];
		//左下、左上、右上、右下
		rectTrans.GetWorldCorners( corners );
		var screenCorner1 = RectTransformUtility.WorldToScreenPoint( camera, corners[ 1 ] );
		var screenCorner3 = RectTransformUtility.WorldToScreenPoint( camera, corners[ 3 ] );

		//左下基準
		var screenRect = new Rect();
		screenRect.x = screenCorner1.x;
		screenRect.width = screenCorner3.x - screenRect.x;
		screenRect.y = screenCorner3.y;
		screenRect.height = screenCorner1.y - screenRect.y;
#if false
		//左、上、右、下
		var margin = new Vector4( screenCorner1.x, Screen.height - screenCorner1.y, Screen.width - screenCorner3.x, screenCorner3.y );
		Debug.Log("margin x:" + margin.x + "y:" + margin.y + "z:" + margin.z + "w:" + margin.w );
		_WebView.SetMargins( (int)margin.x, (int)margin.y, (int)margin.z, (int)margin.w ); 
#endif
		return screenRect;
	}

	private static Texture2D _debugTexture = null;
	private void OnGUI()
	{
		if( _debugTexture == null )
		{
			_debugTexture = new Texture2D( 4, 4, TextureFormat.ARGB32, false );
			for (int y = 0; y < _debugTexture.height; y++) {
				for (int x = 0; x < _debugTexture.width; x++) {
					Color color = new Color( 1.0f, 0.0f, 0.0f, 0.5f );
					_debugTexture.SetPixel(x, y, color);
				}
			}
			_debugTexture.Apply();
		}
		//こっちは左上基準
		var rect = _screenRect;
		rect.yMax = Screen.height - _screenRect.yMin;
		rect.yMin = rect.yMax - _screenRect.height;
		GUI.DrawTexture( rect, _debugTexture );
	}
}