Update from https://crrev.com/303153

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 25ee0c19df0dad90feb632708e1643e46ab9fcbc..e2e543947a10a97595b828c63a432c7710aaab79 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -291,7 +291,8 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
 class BASE_EXPORT FieldTrialList {
  public:
   // Specifies whether field trials should be activated (marked as "used"), when
-  // created using |CreateTrialsFromString()|.
+  // created using |CreateTrialsFromString()|. Has no effect on trials that are
+  // prefixed with |kActivationMarker|, which will always be activated."
   enum FieldTrialActivationMode {
     DONT_ACTIVATE_TRIALS,
     ACTIVATE_TRIALS,
@@ -302,6 +303,10 @@ class BASE_EXPORT FieldTrialList {
   // second process to mimic our state (i.e., provide the same group name).
   static const char kPersistentStringSeparator;  // Currently a slash.
 
+  // Define a marker character to be used as a prefix to a trial name on the
+  // command line which forces its activation.
+  static const char kActivationMarker;  // Currently an asterisk.
+
   // Year that is guaranteed to not be expired when instantiating a field trial
   // via |FactoryGetFieldTrial()|.  Set to two years from the build date.
   static int kNoExpirationYear;
@@ -412,9 +417,10 @@ class BASE_EXPORT FieldTrialList {
   // used in a non-browser process, to carry randomly selected state in a
   // browser process into this non-browser process, but could also be invoked
   // through a command line argument to the browser process. The created field
-  // trials are marked as "used" for the purposes of active trial reporting if
-  // |mode| is ACTIVATE_TRIALS. Trial names in |ignored_trial_names| are ignored
-  // when parsing |prior_trials|.
+  // trials are all marked as "used" for the purposes of active trial reporting
+  // if |mode| is ACTIVATE_TRIALS, otherwise each trial will be marked as "used"
+  // if it is prefixed with |kActivationMarker|. Trial names in
+  // |ignored_trial_names| are ignored when parsing |prior_trials|.
   static bool CreateTrialsFromString(
       const std::string& prior_trials,
       FieldTrialActivationMode mode,