App launcher: Added Finch experiment for the search ranking algorithm.

ui/app_list/search/mixer.h

Index: ui/app_list/search/mixer.h
diff --git a/ui/app_list/search/mixer.h b/ui/app_list/search/mixer.h
index 1322057545ce60e1465242cac7326781741fa406..ea650d28df327e6895442f2b5540e616a2346f27 100644
--- a/ui/app_list/search/mixer.h
+++ b/ui/app_list/search/mixer.h
@@ -35,15 +35,21 @@ class APP_LIST_EXPORT Mixer {
 
   // Adds a new mixer group. A maximum of |max_results| results will be
   // displayed from this group (if 0, will allow unlimited results from this
-  // group). Each result in the group will have its score boosted by |boost|.
+  // group). Behaviour depends on the AppListMixer field trial:
+  // - If default: Each result in the group will have its score boosted by
+  //   |boost|. |multiplier| is ignored.
+  // - If "Blended": |max_results| is a "soft" maximum; if there aren't enough
+  //   results from all groups, more than |max_results| may be chosen from this
+  //   group. Each result in the group will have its score multiplied by
+  //   |multiplier|. |boost| is ignored.
   // Returns the group's group_id.
-  size_t AddGroup(size_t max_results, double boost);
+  size_t AddGroup(size_t max_results, double boost, double multiplier);
 
   // Adds a new mixer group for the special "omnibox" group. This group will be
   // treated specially by the Mixer (it will be truncated such that it fills the
   // remaining slots without overflowing, but with at least one result). A
   // maximum of one group should be added using this method.
-  size_t AddOmniboxGroup(size_t max_results, double boost);
+  size_t AddOmniboxGroup(size_t max_results, double boost, double multiplier);
 
   // Associates a provider with a mixer group.
   void AddProviderToGroup(size_t group_id, SearchProvider* provider);
@@ -87,8 +93,9 @@ class APP_LIST_EXPORT Mixer {
   Groups groups_;
 
   // The ID of the omnibox group. The group with this ID will be treated
-  // specially by the Mixer.
-  // TODO(mgiuca): Omnibox group should not be treated specially.
+  // specially by the Mixer. Ignored if the AppListMixer field trial is
+  // "Blended".
+  // TODO(mgiuca): Remove this after the field trial is complete.
   size_t omnibox_group_ = 0;
   // Whether |omnibox_group_| has been set.
   bool has_omnibox_group_ = false;