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..243cde33c3f4b635aa63ed69793d2dcca56c79de 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -59,6 +59,7 @@
 #include <vector>
 
 #include "base/base_export.h"
+#include "base/callback_forward.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/observer_list_threadsafe.h"
@@ -289,6 +290,11 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
 // Only one instance of this class exists.
 class BASE_EXPORT FieldTrialList {
  public:
+  // A callback that returns true if |name| describes a field trial that should
+  // be accepted by the method it was passed to.
+  typedef base::Callback<bool(const std::string& trial_name)>
+      IsAcceptedFieldTrialCallback;
+
   // Specifies whether field trials should be activated (marked as "used"), when
   // created using |CreateTrialsFromString()|.
   enum FieldTrialActivationMode {
@@ -412,9 +418,12 @@ 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. If |is_accepted_callback| is not null: field
+  // trials for which |is_accepted_callback| returns false are ignored.
+  static bool CreateTrialsFromString(
+      const std::string& prior_trials,
+      FieldTrialActivationMode mode,
+      const IsAcceptedFieldTrialCallback& is_accepted_callback);
 
   // Create a FieldTrial with the given |name| and using 100% probability for
   // the FieldTrial, force FieldTrial to have the same group string as