Omnibox UI Experiments: Add flag to change max autocomplete matches.

components/omnibox/browser/autocomplete_result.cc

Index: components/omnibox/browser/autocomplete_result.cc
diff --git a/components/omnibox/browser/autocomplete_result.cc b/components/omnibox/browser/autocomplete_result.cc
index bc51bd46845d919461dd968c60bc1d125f6c21d3..ac307661068255c3d039bd8515172f2400610fe7 100644
--- a/components/omnibox/browser/autocomplete_result.cc
+++ b/components/omnibox/browser/autocomplete_result.cc
@@ -9,6 +9,7 @@
 
 #include "base/command_line.h"
 #include "base/logging.h"
+#include "base/metrics/field_trial_params.h"
 #include "base/strings/utf_string_conversions.h"
 #include "components/metrics/proto/omnibox_event.pb.h"
 #include "components/metrics/proto/omnibox_input_type.pb.h"
@@ -21,7 +22,11 @@
 #include "components/url_formatter/url_fixer.h"
 
 // static
-const size_t AutocompleteResult::kMaxMatches = 6;
+size_t AutocompleteResult::GetMaxMatches() {
+  return base::GetFieldTrialParamByFeatureAsInt(
+      omnibox::kUIExperimentMaxAutocompleteMatches,
+      OmniboxFieldTrial::kUIMaxAutocompleteMatchesParam, 6);

[Peter Kasting, 2017/05/10 22:08:05]

Nit: For clarity, pull this 6 out as a named constexpr

[tommycli, 2017/05/10 22:44:29]

Done.

+}
 
 void AutocompleteResult::Selection::Clear() {
   destination_url = GURL();
@@ -31,7 +36,7 @@ void AutocompleteResult::Selection::Clear() {
 
 AutocompleteResult::AutocompleteResult() {
   // Reserve space for the max number of matches we'll show.
-  matches_.reserve(kMaxMatches);
+  matches_.reserve(GetMaxMatches());
 
   // It's probably safe to do this in the initializer list, but there's little
   // penalty to doing it here and it ensures our object is fully constructed
@@ -129,8 +134,8 @@ void AutocompleteResult::SortAndCull(
 
   SortAndDedupMatches(input.current_page_classification(), &matches_);
 
-  // Sort and trim to the most relevant kMaxMatches matches.
-  size_t max_num_matches = std::min(kMaxMatches, matches_.size());
+  // Sort and trim to the most relevant GetMaxMatches() matches.
+  size_t max_num_matches = std::min(GetMaxMatches(), matches_.size());
   CompareWithDemoteByType<AutocompleteMatch> comparing_object(
       input.current_page_classification());
   std::sort(matches_.begin(), matches_.end(), comparing_object);