Add code to FieldTrialList to help diagnose a renderer crash.

We're seeing inconsistencies between what state renderers received
on the command-line vs. what they later get via IPC. This change
adds checks on the browser side that would detect when this happens
- by keeping a map of the field trial states that were sent on the
command-line or sent via IPC and CHECK'ing when there's a mismatch.
This will hopefully provide a stack trace when this happens on the
browser process.

BUG=359406

Review URL: https://codereview.chromium.org/1693703003

Cr-Commit-Position: refs/heads/master@{#375223}

2 files changed, 17 insertions, 0 deletions

diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index 866c53a..647fe55 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -105,6 +105,16 @@ bool ParseFieldTrialsString(const std::string& trials_string,
   return true;
 }
 
+void CheckTrialGroup(const std::string& trial_name,
+                     const std::string& trial_group,
+                     std::map<std::string, std::string>* seen_states) {
+  if (ContainsKey(*seen_states, trial_name)) {
+    CHECK_EQ((*seen_states)[trial_name], trial_group) << trial_name;
+  } else {
+    (*seen_states)[trial_name] = trial_group;
+  }
+}
+
 }  // namespace
 
 // statics
@@ -475,6 +485,9 @@ void FieldTrialList::AllStatesToString(std::string* output) {
     output->append(1, kPersistentStringSeparator);
     trial.group_name.AppendToString(output);
     output->append(1, kPersistentStringSeparator);
+
+    CheckTrialGroup(trial.trial_name.as_string(), trial.group_name.as_string(),
+                    &global_->seen_states_);
   }
 }
 
@@ -600,6 +613,8 @@ void FieldTrialList::NotifyFieldTrialGroupSelection(FieldTrial* field_trial) {
   if (!field_trial->enable_field_trial_)
     return;
 
+  CheckTrialGroup(field_trial->trial_name(), field_trial->group_name_internal(),
+                  &global_->seen_states_);
   global_->observer_list_->Notify(
       FROM_HERE, &FieldTrialList::Observer::OnFieldTrialGroupFinalized,
       field_trial->trial_name(), field_trial->group_name_internal());
diff --git a/base/metrics/field_trial.h b/base/metrics/field_trial.h
index 7bfc1de..0dfa036 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -519,6 +519,8 @@ class BASE_EXPORT FieldTrialList {
   base::Lock lock_;
   RegistrationMap registered_;
 
+  std::map<std::string, std::string> seen_states_;
+
   // Entropy provider to be used for one-time randomized field trials. If NULL,
   // one-time randomization is not supported.
   scoped_ptr<const FieldTrial::EntropyProvider> entropy_provider_;