Initial implementation for sharing field trial state (win)

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 2b8894904dd350e94899581bd858506300e83cd7..3851cd8639ebb9763b0b8154dbfbfb51c3869a10 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -64,9 +64,11 @@
 #include <vector>
 
 #include "base/base_export.h"
+#include "base/command_line.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/shared_memory.h"
 #include "base/observer_list_threadsafe.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
@@ -464,6 +466,26 @@ class BASE_EXPORT FieldTrialList {
       const std::string& trials_string,
       const std::set<std::string>& ignored_trial_names);
 
+  // If using shared memory to pass around the list of field trials, then

[Alexei Svitkine (slow), 2016/10/06 13:45:30]

The first sentence should describe what the function does. Then, you can go into
details of how the args work.

[lawrencewu, 2016/10/06 20:14:57]

Done.

+  // expects |field_trial_handle_switch| argument to
+  // contain the shared memory handle, and it opens the handle, reads the memory
+  // and creates the trials.
+  // If not, then create the trials as before (using the kForceFieldTrials
+  // switch). Needs the |field_trial_handle_switch| argument to be passed in
+  // since base/ can't depend on content.
+  static void CreateTrialsFromCommandLine(
+      const base::CommandLine cmd_line,

[Alexei Svitkine (slow), 2016/10/06 13:45:30]

Pass by const& - otherwise you're copying the object. Here and in the .cc file.

[lawrencewu, 2016/10/06 20:14:57]

Done.

+      const char* field_trial_handle_switch);
+
+  // 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> CopyFieldTrialStateToSharedMemory(
+      base::CommandLine* cmd_line,
+      const char* field_trial_handle_switch);
+
   // Create a FieldTrial with the given |name| and using 100% probability for
   // the FieldTrial, force FieldTrial to have the same group string as
   // |group_name|. This is commonly used in a non-browser process, to carry