Update OfflinePagesPrefetchingSuggestions fieldtrial config

So that the enabled group name matches the upcoming new name for the
experiment configuration.

Bug: 687364
Change-Id: I4798aefb08f2531eaabd74ca04d7b870f5d25a88
Reviewed-on: https://chromium-review.googlesource.com/1105183Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568265}

testing/variations/fieldtrial_testing_config.json

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