Add metric Omnibox.FocusToOpenTime for time from omnibox focus to omnibox usage.

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 9306 matching lines @@
   </summary>
 </histogram>
 
 <histogram name="Omnibox.FocusToEditTime" units="ms">
   <summary>
     The length of time between when a user focused on the omnibox and first
     modifies the omnibox.
   </summary>
 </histogram>
 
+<histogram name="Omnibox.FocusToOpenTime" units="ms">
+  <summary>
+    The length of time between when a user focused on the omnibox and opened an
+    omnibox match (which could be what they typed or a suggestion).
+  </summary>
+</histogram>
+
 <histogram name="Omnibox.ProviderTime" units="ms">
   <summary>
     The length of time taken by the named provider&quot;s synchronous pass.
   </summary>
 </histogram>
 
 <histogram name="Omnibox.QueryBookmarksTime">
   <obsolete>
     Deprecated 2012-11-14. Replaced by Autocomplete.BookmarkProviderMatchTime.
   </obsolete>
@@ skipping 15002 matching lines @@
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>
   <group name="PlusIcon" label="Plus icon in apps page"/>
   <affected-histogram name="Extensions.AppLaunch"/>
   <affected-histogram name="NewTabPage.DefaultPageType"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>