Add flag enabling 0-prefix onboarding suggestion.

This flag will allow us to work and experiment with onboarding
suggestions for new users.

Related CL: http://cl/285006555

Bug: 1050831
Change-Id: Ie6de46f043c50857098a7ee66097b08a2eb6dbc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2048765
Commit-Queue: Ender <ender@google.com>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742264}

chrome/browser/about_flags.cc

@@ -924,7 +924,9 @@ const FeatureEntry::FeatureVariation kOmniboxOnFocusSuggestionsVariations[] = {
     {"ZPS on NTP: Local History", kOmniboxNTPZPSLocal,
      base::size(kOmniboxNTPZPSLocal), nullptr},
     {"ZPS on NTP: Remote History", kOmniboxNTPZPSRemote,
-     base::size(kOmniboxNTPZPSRemote), "t3314248"},
+     base::size(kOmniboxNTPZPSRemote), /* variation_id */ "t3314248"},
+    {"ZPS on NTP: Onboarding", kOmniboxNTPZPSRemote,
+     base::size(kOmniboxNTPZPSRemote), /* variation_id */ "t3316638"},
 #else   // !defined(OS_ANDROID)
     {"NTP Omnibox - Remote,Local",
      kOmniboxOnFocusSuggestionsParamNTPOmniboxRemoteLocal,