Introduce a new FieldTrialList::IsTrialActive() API.

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
diff --git a/base/metrics/field_trial.h b/base/metrics/field_trial.h
index 93521288b7af4386a2a370b8b8e8658c852a6c20..c499cc53e3bc63896d856d4c8f1c00ea32552d06 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -413,6 +413,9 @@ class BASE_EXPORT FieldTrialList {
   // Returns true if the named trial has been registered.
   static bool TrialExists(const std::string& name);
 
+  // Returns true if the named trial exists and has been activated.
+  static bool IsTrialActive(const std::string& name);
+
   // Creates a persistent representation of active FieldTrial instances for
   // resurrection in another process. This allows randomization to be done in
   // one process, and secondary processes can be synchronized on the result.