Omnibox UI Experiments: Add vertical margin variations to about:flags

components/omnibox/browser/omnibox_field_trial.h

Index: components/omnibox/browser/omnibox_field_trial.h
diff --git a/components/omnibox/browser/omnibox_field_trial.h b/components/omnibox/browser/omnibox_field_trial.h
index 2ff41730ad4f354098ebb40963a67a9c3b903033..1bff94e1f2aa754402e18bd36669526db08d2a47 100644
--- a/components/omnibox/browser/omnibox_field_trial.h
+++ b/components/omnibox/browser/omnibox_field_trial.h
@@ -32,6 +32,7 @@ extern const base::Feature kSearchProviderContextAllowHttpsUrls;
 extern const base::Feature kZeroSuggestRedirectToChrome;
 extern const base::Feature kZeroSuggestSwapTitleAndUrl;
 extern const base::Feature kDisplayTitleForCurrentUrl;
+extern const base::Feature kUIExperiments;

[Mark P, 2017/05/03 18:38:59]

Double-check:
Please make sure that if we make this a feature, then we can independent
about:flags selectors that are both tied to the same feature, just set different
parameters.

[tommycli, 2017/05/03 18:49:46]

Done. I just tested this (one Feature, multiple about:flag selectors), and it
seems to work.

Admittedly, it's unprecedented.

I would say -- if indeed it doesn't work later on down the line, it wouldn't be
too disruptive in the code to split it into separate base::Feature instances.

 }
 
 // The set of parameters customizing the HUP scoring.
@@ -495,6 +496,9 @@ class OmniboxFieldTrial {
   static const char kZeroSuggestRedirectToChromeServerAddressParam[];
   static const char kZeroSuggestRedirectToChromeAdditionalFieldsParam[];
 
+  // Parameter names used by UI experiments.
+  static const char kUIExperimentsVerticalMarginParam[];
+
   // The amount of time to wait before sending a new suggest request after the
   // previous one unless overridden by a field trial parameter.
   // Non-const because some unittests modify this value.