[omnibox] add omnibox on device head suggest BETA experiments to

fieldtrial_testing_config.json.

Note these experiments are for desktop and Android. iOSs will be
added in a different study "OmniboxOnDeviceHeadSuggestIOS"

Bug: 925072
Change-Id: I557fc1f91fb9e12283bb62f957eee5dd3146f054
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872942Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Ce Chen <cch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708366}

testing/variations/fieldtrial_testing_config.json

@@ -4056,6 +4056,25 @@
             ]
         }
     ],
+    "OmniboxOnDeviceHeadSuggest": [
+        {
+            "platforms": [
+                "android",
+                "windows",
+                "mac",
+                "chromeos",
+                "linux"
+            ],
+            "experiments": [
+                {
+                    "name": "Beta_OnDeviceHeadSuggest_Enabled",
+                    "enable_features": [
+                        "OmniboxOnDeviceHeadProvider"
+                    ]
+                }
+            ]
+        }
+    ],
     "OmniboxSteadyStateElisions": [
         {
             "platforms": [