Make some field trials unforceable via command-line in the official build.

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 5a4375ee65ed1510b00b7062b0adae735ab3d794..51177eb93f7b8a1173b30878146dbe45e641b442 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -55,6 +55,7 @@
 #define BASE_METRICS_FIELD_TRIAL_H_
 
 #include <map>
+#include <set>
 #include <string>
 #include <vector>
 
@@ -412,9 +413,12 @@ class BASE_EXPORT FieldTrialList {
   // 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.
-  static bool CreateTrialsFromString(const std::string& prior_trials,
-                                     FieldTrialActivationMode mode);
+  // |mode| is ACTIVATE_TRIALS. Trial names in |ignored_trial_names| are ignored
+  // when parsing |prior_trials|.
+  static bool CreateTrialsFromString(
+      const std::string& prior_trials,
+      FieldTrialActivationMode mode,
+      const std::set<std::string>& ignored_trial_names);
 
   // Create a FieldTrial with the given |name| and using 100% probability for
   // the FieldTrial, force FieldTrial to have the same group string as