Adds field trial testing config for trending zero-prefix suggestions

Canary experiment: cl/330845211
Beta experiment: cl/335678210

Bug: 1122669
Change-Id: I2a2187c4d9e7ac1107f1c4bab20b79f9ca10b698
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453434
Auto-Submit: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815274}

testing/variations/fieldtrial_testing_config.json

@@ -4662,6 +4662,7 @@
                         "OmniboxReactiveZeroSuggestionsOnNTPRealbox",
                         "OmniboxRemoveSuggestionsFromClipboard",
                         "OmniboxRichEntitySuggestions",
+                        "OmniboxTrendingZeroPrefixSuggestionsOnNTP",
                         "OmniboxUIExperimentMaxAutocompleteMatches",
                         "QueryInOmnibox",
                         "SearchSuggestChips"