[omnibox] Add GOC Variations to about:flags for on-clobber

Previous CL added GOC Variations for on-focus:
https://chromium-review.googlesource.com/c/chromium/src/+/2422473

This CL adds the exact same variations for on-clobber.

It also makes the on-clobber Desktop only, as we never implemented the
UI hook for Mobile (either Android or iOS).

Bug: 1106096
Change-Id: If5cc05667e86892b8d491777e950e37ef81a3f8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424545
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Auto-Submit: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809485}

chrome/browser/about_flags.cc

@@ -3718,8 +3718,12 @@ const FeatureEntry kFeatureEntries[] = {
      flag_descriptions::kOmniboxClobberTriggersContextualWebZeroSuggestName,
      flag_descriptions::
          kOmniboxClobberTriggersContextualWebZeroSuggestDescription,
-     kOsAll,
-     FEATURE_VALUE_TYPE(omnibox::kClobberTriggersContextualWebZeroSuggest)},
+     kOsDesktop,
+     FEATURE_WITH_PARAMS_VALUE_TYPE(
+         omnibox::kClobberTriggersContextualWebZeroSuggest,
+         // On-clobber has the same variations and forcing IDs as on-focus.
+         kOmniboxOnFocusSuggestionsContextualWebVariations,
+         "OmniboxGoogleOnContent")},
 
     {"omnibox-on-device-head-suggestions-incognito",
      flag_descriptions::kOmniboxOnDeviceHeadSuggestionsIncognitoName,