Update iOS Desktop User Agent string to Safari 11.1.1.

Bug: 852276
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I4313bff438d9c8a0d05bf995bb0025040c9fe5d9
Reviewed-on: https://chromium-review.googlesource.com/1099175
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566965}

ios/chrome/browser/ui/chrome_web_view_factory.mm

@@ -25,10 +25,10 @@ NSString* const kExternalUserAgent = @"UIWebViewForExternalContent";
 namespace ChromeWebView {
 // Shared desktop user agent used to mimic Safari on a mac.
 NSString* const kDesktopUserAgent =
-    @"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_4) "
-    @"AppleWebKit/600.7.12 (KHTML, like Gecko) "
-    @"Version/8.0.7 "
-    @"Safari/600.7.12";
+    @"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_5) "
+    @"AppleWebKit/605.1.15 (KHTML, like Gecko) "
+    @"Version/11.1.1 "
+    @"Safari/605.1.15";
 
 NSString* const kExternalRequestGroupID = @"kExternalRequestGroupID";