No Issue - Add navigator.webdriver attribute.

This patch adds an enumerable, configurable, readonly attribute "webdriver" to
the Navigator object. The attribute is always false because we do not support
WebDriver or scripted automation.

The navigator.webdriver attribute is meant as an indication to web authors that
a document is visited by WebDriver. It is important to stress that it is not
meant as a way to detect that a website is being visited by a browser automation
tool or bot, but as a tool for web documents to take alternate code paths.

On the off-chance exposing navigator.webdriver turns out to be catastrophic,
we put it behind a new preference dom.webdriver.enabled that controls its
exposure.

dom/webidl/Navigator.webidl

@@ -12,6 +12,7 @@
  * https://dvcs.w3.org/hg/gamepad/raw-file/default/gamepad.html#navigator-interface-extension
  * http://www.w3.org/TR/beacon/#sec-beacon-method
  * https://html.spec.whatwg.org/#navigatorconcurrenthardware
+ * https://w3c.github.io/webdriver/webdriver-spec.html#interface
  *
  * © Copyright 2004-2020 Apple Computer, Inc., Mozilla Foundation,
  * Opera Software ASA and Moonchild Productions. You are granted a license to use,
@@ -31,6 +32,7 @@ Navigator implements NavigatorStorageUtils;
 Navigator implements NavigatorConcurrentHardware;
 Navigator implements NavigatorStorage;
 Navigator implements NavigatorGlobalPrivacyControl;
+Navigator implements NavigatorAutomationInformation;
 
 [NoInterfaceObject, Exposed=(Window,Worker)]
 interface NavigatorID {
@@ -334,3 +336,10 @@ partial interface Navigator {
   [Pref="dom.events.asyncClipboard", SecureContext, SameObject]
   readonly attribute Clipboard clipboard;
 };
+
+// https://w3c.github.io/webdriver/webdriver-spec.html#interface
+[NoInterfaceObject, Exposed=Window]
+interface NavigatorAutomationInformation {
+  [Pref="dom.webdriver.enabled"]
+  readonly attribute boolean webdriver;
+};