[omnibox] Add OmniboxDisableInstantExtendedLimit to iOS testing config.

Change-Id: I743f0fd55842f9f3aa9f3732667cfc950333fab8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2229464Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775099}

testing/variations/fieldtrial_testing_config.json

@@ -4131,6 +4131,7 @@
                         "ZeroSuggestVariant:8:*": "RemoteNoUrl"
                     },
                     "enable_features": [
+                        "OmniboxDisableInstantExtendedLimit",
                         "OmniboxHistoryQuickProviderAllowButDoNotScoreMidwordTerms",
                         "OmniboxHistoryQuickProviderAllowMidwordContinuations",
                         "OmniboxOnFocusSuggestions"