Add Trending Queries to testing config.

This change ensures our trybots run the code with the
OmniboxTrendingZeroPrefixSuggestionsOnNTP feature flag
enabled.

Bug: 1122669
Change-Id: Ie232eb0328f6f5b343a6184997189e5a68231391
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515201
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823764}

testing/variations/fieldtrial_testing_config.json

@@ -4705,6 +4705,7 @@
                         "OmniboxMaxURLMatches",
                         "OmniboxMaxZeroSuggestMatches",
                         "OmniboxOnFocusSuggestionsContextualWebOnContent",
+                        "OmniboxTrendingZeroPrefixSuggestionsOnNTP",
                         "OmniboxUIExperimentMaxAutocompleteMatches"
                     ]
                 }