Made web view preloading code and scheme handler shared code

Multiplatform/Shared/Article/WrapperScriptMessageHandler.swift

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+//
+//  WrapperScriptMessageHandler.swift
+//  Multiplatform iOS
+//
+//  Created by Maurice Parker on 7/6/20.
+//  Copyright © 2020 Ranchero Software. All rights reserved.
+//
+
+import Foundation
+import WebKit
+
+class WrapperScriptMessageHandler: NSObject, WKScriptMessageHandler {
+	
+	// We need to wrap a message handler to prevent a circlular reference
+	private weak var handler: WKScriptMessageHandler?
+	
+	init(_ handler: WKScriptMessageHandler) {
+		self.handler = handler
+	}
+	
+	func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
+		handler?.userContentController(userContentController, didReceive: message)
+	}
+	
+}