[ios] Enable return to regular tabs when closing last tab

Enable the experiment to return to regular tabs when closing last tab.

Bug: 891396
Change-Id: I0a0784c901fc0f19508fdf5db612c350ed8d38ef
Reviewed-on: https://chromium-review.googlesource.com/c/1297177Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602478}

ios/chrome/browser/ui/ui_feature_flags.cc

@@ -9,7 +9,7 @@ const base::Feature kFirstResponderKeyWindow{"FirstResponderKeyWindow",
 
 // TODO(crbug.com/893314) : Remove this flag.
 const base::Feature kClosingLastIncognitoTab{"ClosingLastIncognitoTab",
-                                             base::FEATURE_DISABLED_BY_DEFAULT};
+                                             base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kBrowserContainerContainsNTP{
     "BrowserContainerContainsNTP", base::FEATURE_DISABLED_BY_DEFAULT};