Added a SetForced method to allow forcing a new set of group bucketting.

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index 245f2bf02a6a6e07c4ff0f625c91397f2c04a657..6d4f2eb06b2d47c28dde9c2389e68d4814551cae 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -170,6 +170,21 @@ void FieldTrial::EnableBenchmarking() {
   enable_benchmarking_ = true;
 }
 
+void FieldTrial::SetForced() {
+  // We might have been forced before (e.g., by CreateFieldTrial) and it's
+  // first come first served, e.g., command line switch has precedence.
+  if (forced_)
+    return;
+  // Explicit forcing should only be for cases where we want to set the group
+  // probabilities before the hard coded field trial setup is executed. So
+  // there must have been at least one non-default group appended at that point.
+  DCHECK_GT(next_group_number_, kDefaultGroupNumber + 1);
+
+  // And we must finalize the group choice before we mark ourselves as forced.
+  group();
+  forced_ = true;
+}
+
 FieldTrial::~FieldTrial() {}
 
 void FieldTrial::SetGroupChoice(const std::string& name, int number) {