Enable PresentSadTabInViewController feature by default.

Bug: 901563
Test:
 - SadTab is correctly presented for renderer crashes and chrome://crash
 - SadTab support pull to action
 - SadTab show different help text for incognito
 - SadTab bottons and links work as expected (reload, help page,
   feedback)
 - SadTab is correctly displayed in TabGrid and during SideSwipe
 - The user can switch between tabs and that does not break SadTab UI

Change-Id: I68a7a972a97375f4aff084aa83095a736c8e3b41
Reviewed-on: https://chromium-review.googlesource.com/c/1332775
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609349}

ios/chrome/browser/ui/sad_tab/features.cc

@@ -5,4 +5,4 @@
 #include "ios/chrome/browser/ui/sad_tab/features.h"
 
 const base::Feature kPresentSadTabInViewController{
-    "PresentSadTabInViewController", base::FEATURE_DISABLED_BY_DEFAULT};
+    "PresentSadTabInViewController", base::FEATURE_ENABLED_BY_DEFAULT};