[omnibox] Add test config for local entity feature.

Bug: 950512
Change-Id: Icfcd010a8d245b36c6cde9260a41f6f5d7170342
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632425Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Nik Bhagat <nikunjb@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664743}

testing/variations/fieldtrial_testing_config.json

@@ -3513,6 +3513,7 @@
                     "enable_features": [
                         "OmniboxDisplayTitleForCurrentUrl",
                         "OmniboxDocumentProvider",
+                        "OmniboxLocalEntitySuggestions",
                         "OmniboxRichEntitySuggestions",
                         "OmniboxTailSuggestions",
                         "OmniboxUIExperimentMaxAutocompleteMatches",