Add variations params api to get params from features.

base/feature_list.h

Index: base/feature_list.h
diff --git a/base/feature_list.h b/base/feature_list.h
index 44aeb4b88b7e531eb3b402724578789e471dc1ad..2a47427fb268e6d63cba9005fb31727ed326aa2f 100644
--- a/base/feature_list.h
+++ b/base/feature_list.h
@@ -141,6 +141,10 @@ class BASE_EXPORT FeatureList {
   // struct, which is checked in builds with DCHECKs enabled.
   static bool IsEnabled(const Feature& feature);
 
+  // Returns the field trial associated with the given |feature|. Must only be
+  // called after the singleton instance has been registered via SetInstance().
+  static FieldTrial* GetFieldTrial(const Feature& feature);

[Alexei Svitkine (slow), 2016/05/03 17:04:25]

Can you also add a unit test for this API?

[jwd, 2016/05/03 20:12:27]

Done.

+
   // Splits a comma-separated string containing feature names into a vector.
   static std::vector<std::string> SplitFeatureListString(
       const std::string& input);
@@ -200,6 +204,12 @@ class BASE_EXPORT FeatureList {
   // Requires the FeatureList to have already been fully initialized.
   bool IsFeatureEnabled(const Feature& feature);
 
+  // Returns the field trial associated with the given |feature|. This is
+  // invoked by the public FeatureList::GetFieldTrial() static function on the
+  // global singleton. Requires the FeatureList to have already been fully
+  // initialized.
+  base::FieldTrial* GetAssociatedFieldTrial(const Feature& feature);
+
   // For each feature name in comma-separated list of strings |feature_list|,
   // registers an override with the specified |overridden_state|. Also, will
   // associate an optional named field trial if the entry is of the format