[omnibox] Add new feature experiments to the fieldtrial testing config.

Note that I'm omitting the URL elision experiment
(OmniboxUIExperimentHideSteadyStateUrlSchemeAndSubdomains). That seems
to have broken a few browser tests, so tommycli is going to fix them
and add it to the testing config in a later CL.

Bug: 854714, 806995, 824893

Change-Id: I99b79d22ee5f1a3df0ec780a4283c7211f107a12
Reviewed-on: https://chromium-review.googlesource.com/1176391Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584065}

testing/variations/fieldtrial_testing_config.json

@@ -2856,6 +2856,9 @@
                     "enable_features": [
                         "OmniboxBreakWordsAtUnderscores",
                         "OmniboxDisplayTitleForCurrentUrl",
+                        "OmniboxNewAnswerLayout",
+                        "OmniboxRichEntitySuggestions",
+                        "OmniboxTabSwitchSuggestions",
                         "OmniboxTailSuggestions",
                         "OmniboxUIExperimentShowSuggestionFavicons",
                         "OmniboxUIExperimentSwapTitleAndUrl",