[omnibox] Add field-trial test config for max suggestions/URLs

Just adds a field-trial test config to exercise the higher suggestions,
URL limit logic. Desktop only.

(Also removes mention of OmniboxTailSuggestions.)

Bug: 964049
Change-Id: Iad68a65c59a1033887ea41a3f6f4efb444052a77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1778361
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693736}

testing/variations/fieldtrial_testing_config.json

@@ -3883,12 +3883,16 @@
             "experiments": [
                 {
                     "name": "DesktopExperiments",
+                    "params": {
+                        "OmniboxMaxURLMatches": "7",
+                        "UIMaxAutocompleteMatches": "8"
+                    },
                     "enable_features": [
                         "OmniboxDisplayTitleForCurrentUrl",
                         "OmniboxDocumentProvider",
                         "OmniboxLocalEntitySuggestions",
+                        "OmniboxMaxURLMatches",
                         "OmniboxRichEntitySuggestions",
-                        "OmniboxTailSuggestions",
                         "OmniboxUIExperimentMaxAutocompleteMatches",
                         "QueryInOmnibox"
                     ]