Added protection for history.length

As requested by #231.

lib/modifiedHistoryAPI.js

@@ -0,0 +1,56 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+(function(){
+	"use strict";
+	
+	var scope;
+	if ((typeof exports) !== "undefined"){
+		scope = exports;
+	}
+	else {
+		window.scope.modifiedHistoryAPI = {};
+		scope = window.scope.modifiedHistoryAPI;
+	}
+	
+	const {hasType, checkerWrapper} = require("./modifiedAPIFunctions");
+	
+	scope.changedGetters = [
+		{
+			objectGetters: [function(window){return window.History.prototype;}],
+			name: "length",
+			getterGenerator: function(checker){
+				const temp = {
+					get length(){
+						return checkerWrapper(checker, this, arguments, function(args, check){
+							const {prefs, notify, window, original} = check;
+							const originalLength = original.apply(this, window.Array.from(args));
+							const threshold = prefs("historyLengthThreshold", window.location);
+							if (originalLength > threshold){
+								notify("fakedHistoryReadout");
+								return threshold;
+							}
+							else {
+								return originalLength;
+							}
+						});
+					}
+				};
+				return Object.getOwnPropertyDescriptor(temp, "length").get;
+			}
+		}
+	];
+	
+	
+	function getStatus(obj, status){
+		status = Object.create(status);
+		status.active = hasType(status, "readout");
+		return status;
+	}
+	
+	scope.changedGetters.forEach(function(changedGetter){
+		changedGetter.type = "readout";
+		changedGetter.getStatus = getStatus;
+		changedGetter.api = "history";
+	});
+}());