Update OfflinePagesPrefetchingSuggestions fieldtrial config again.

The group was finally renamed in the experiment configuration so
renaming it here again.

Bug: 687364
Change-Id: I314b2993f27342054bbb423c80e16d2e7a8f886f
Reviewed-on: https://chromium-review.googlesource.com/1105390Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568472}

testing/variations/fieldtrial_testing_config.json

@@ -2475,7 +2475,7 @@
             ],
             "experiments": [
                 {
-                    "name": "Enabled3",
+                    "name": "Enabled_Holdback",
                     "enable_features": [
                         "KeepPrefetchedContentSuggestions",
                         "OfflinePagesPrefetching"