Add histogram Omnibox.FocusToEditTime for time in milliseconds between a user focus and edit.

chrome/browser/ui/omnibox/omnibox_edit_model.h

Index: chrome/browser/ui/omnibox/omnibox_edit_model.h
===================================================================
--- chrome/browser/ui/omnibox/omnibox_edit_model.h	(revision 209999)
+++ chrome/browser/ui/omnibox/omnibox_edit_model.h	(working copy)
@@ -444,6 +444,10 @@
   // autocomplete text that has not yet been accepted.
   string16 user_text_;
 
+  // We keep track of when the user last focused on the omnibox, but reset it
+  // to null when user input occurs.
+  base::TimeTicks last_omnibox_focus_without_user_input_;
+
   // We keep track of when the user began modifying the omnibox text.
   // This should be valid whenever user_input_in_progress_ is true.
   base::TimeTicks time_user_first_modified_omnibox_;