Mark NavigatorAutomationInformation attributes as non-configurable

Context: go/navigator-webdriver-legacyunforgeable

Change-Id: I5adcc41f519f6b97a88f2f9c74fe6db094b60b81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2460744
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816257}

third_party/blink/renderer/core/frame/navigator_automation_information.idl

@@ -7,5 +7,5 @@
 [
     RuntimeEnabled=AutomationControlled
 ] interface mixin NavigatorAutomationInformation {
-    readonly attribute boolean webdriver;
+    [LegacyUnforgeable] readonly attribute boolean webdriver;
 };

third_party/blink/web_tests/navigator_webdriver/navigator_webdriver_enabled.html

@@ -20,9 +20,9 @@ test(function() {
 
 
 test(function() {
-	var descriptor = Object.getOwnPropertyDescriptor(Object.getPrototypeOf(navigator), "webdriver");
+  var descriptor = Object.getOwnPropertyDescriptor(navigator, "webdriver");
   assert_true(descriptor !== undefined);
-  assert_true(descriptor.configurable);
+  assert_false(descriptor.configurable);
   assert_true(descriptor.enumerable);
   assert_true(descriptor.set === undefined);
 }, "Test that the navigator.webdriver descriptor has expected properties");