[omnibox] Add on-focus feature to the desktop field trial testing config

Change-Id: Id6ca93ae981f128896138524891f8d1b1615e6a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518713Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823810}

testing/variations/fieldtrial_testing_config.json

@@ -4673,6 +4673,7 @@
                         "OmniboxLocalZeroSuggestFrecencyRanking",
                         "OmniboxMaxURLMatches",
                         "OmniboxMaxZeroSuggestMatches",
+                        "OmniboxOnFocusSuggestionsContextualWebOnContent",
                         "OmniboxReactiveZeroSuggestionsOnNTPOmnibox",
                         "OmniboxReactiveZeroSuggestionsOnNTPRealbox",
                         "OmniboxRemoveSuggestionsFromClipboard",