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

This CL implements it on Views only. There will be a followup to do
the variation on Mac.

BUG=717777

Review-Url: https://codereview.chromium.org/2854263002
Cr-Commit-Position: refs/heads/master@{#470050}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 3074292..ee6ba39 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -3359,6 +3359,12 @@
 // for current URL.
 extern const char kOmniboxDisplayTitleForCurrentUrlDescription[];
 
+// Name of the Omnibox UI vertical margin flag.
+extern const char kOmniboxUIVerticalMarginName[];
+
+// Description of the Omnibox UI vertical margin flag.
+extern const char kOmniboxUIVerticalMarginDescription[];
+
 // Name of the flag that forces Network Quality Estimator (NQE) to always
 // return the specified effective connection type.
 extern const char kForceEffectiveConnectionTypeName[];