Omnibox Metrics: Make |completed_length| field sane

components/metrics/proto/omnibox_event.proto

 // Copyright 2014 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.
 //
 // Stores information about an omnibox interaction.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
@@ skipping 37 matching lines @@
   // dropdown is closed when logging these.
   optional bool is_popup_open = 15;
 
   // True if this is a paste-and-search or paste-and-go action.  (The codebase
   // refers to both these types as paste-and-go.)
   optional bool is_paste_and_go = 16;
 
   // The length of the inline autocomplete text in the omnibox.
   // The sum |typed_length| + |completed_length| gives the full length of the
   // user-visible text in the omnibox.
-  // This field is only set for inlineable suggestions selected at position 0
-  // (|selected_index| = 0) and will be omitted otherwise.
+  // This field is only set for suggestions that are allowed to be the default
+  // match and omitted otherwise.  The first suggestion is always allowed to
+  // be the default match.  (This is an enforced constraint.)  Hence, if
+  // |selected_index| == 0, then this field will always be set.
   optional int32 completed_length = 6;
 
   // The amount of time, in milliseconds, since the user first began modifying
   // the text in the omnibox.  If at some point after modifying the text, the
   // user reverts the modifications (thus seeing the current web page's URL
   // again), then writes in the omnibox again, this elapsed time should start
   // from the time of the second series of modification.
   optional int64 typing_duration_ms = 7;
 
   // The amount of time, in milliseconds, since the last time the default
@@ skipping 189 matching lines @@
     // The number of times this provider returned a non-zero number of
     // suggestions during this omnibox session.
     // Note that each provider may define a session differently for its
     // purposes.
     optional int32 times_returned_results_in_session = 5;
   }
   // A list of diagnostic information about each provider.  Providers
   // will appear at most once in this list.
   repeated ProviderInfo provider_info = 12;
 }