Use SharedPersistentMemoryAllocator to share field trial state

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 99e705a1fa82dd80a34701cbad08e857c589d9f7..c1407f9bc34db965cea113eda1e0ab1713200ebc 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -69,7 +69,9 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/shared_memory.h"
+#include "base/metrics/persistent_memory_allocator.h"
 #include "base/observer_list_threadsafe.h"
+#include "base/process/launch.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
@@ -479,14 +481,18 @@ class BASE_EXPORT FieldTrialList {
       const base::CommandLine& cmd_line,
       const char* field_trial_handle_switch);
 
+#if defined(OS_WIN)
+  static void AppendFieldTrialHandleIfNeeded(
+      base::HandlesToInheritVector& handles);
+#endif
+
   // Adds a switch to the command line containing the field trial state as a
   // string (if not using shared memory to share field trial state), or the
   // shared memory handle + length.
   // Needs the |field_trial_handle_switch| argument to be passed in since base/
   // can't depend on content/.
-  static std::unique_ptr<base::SharedMemory> CopyFieldTrialStateToFlags(
-      const char* field_trial_handle_switch,
-      base::CommandLine* cmd_line);
+  static void CopyFieldTrialStateToFlags(const char* field_trial_handle_switch,
+                                         base::CommandLine* cmd_line);
 
   // Create a FieldTrial with the given |name| and using 100% probability for
   // the FieldTrial, force FieldTrial to have the same group string as
@@ -508,10 +514,26 @@ class BASE_EXPORT FieldTrialList {
   // Notify all observers that a group has been finalized for |field_trial|.
   static void NotifyFieldTrialGroupSelection(FieldTrial* field_trial);
 
+  // Called in NotifyFieldTrialGroupSelection(). Updates field_trial_allocator
+  // by adding the newly-created |field_trial|.
+  static void UpdateFieldTrialAllocator(FieldTrial* field_trial);
+
   // Return the number of active field trials.
   static size_t GetFieldTrialCount();
 
  private:
+  // Expects a mapped piece of shared memory |shm| that was created from the
+  // browser process's field_trial_allocator and shared via the command line.
+  // This function recreates the allocator, iterates through all the field
+  // trials in it, and creates them via CreateFieldTrial().
+  static void CreateTrialsFromSharedMemory(
+      std::unique_ptr<base::SharedMemory> shm);
+
+  // Allocator used to instantiate field trial in child processes. In the
+  // future, we may want to move this to a more generic place if we want to
+  // start passing more data other than field trials.
+  static base::SharedPersistentMemoryAllocator* field_trial_allocator;
+
   // A map from FieldTrial names to the actual instances.
   typedef std::map<std::string, FieldTrial*> RegistrationMap;