Optimize a couple more code paths in feature params look up code.

Makes GetFieldTrialParamValueByFeature() not do an extra field
trial look up by making it match the GetFieldTrialParamValue()
code path.

Also merges GetFieldTrialParamsWithoutFallback()'s contains and
map access into a single lookup, rather than two.

Bug: 1163921
Change-Id: I602e0ee47f679b143ee1666923b53a2760cae6c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617470
Auto-Submit: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842188}

base/metrics/field_trial_param_associator.cc

@@ -60,10 +60,11 @@ bool FieldTrialParamAssociator::GetFieldTrialParamsWithoutFallback(
   AutoLock scoped_lock(lock_);
 
   const FieldTrialRefKey key(trial_name, group_name);
-  if (!Contains(field_trial_params_, key))
+  auto it = field_trial_params_.find(key);
+  if (it == field_trial_params_.end())
     return false;
 
-  *params = field_trial_params_[key];
+  *params = it->second;
   return true;
 }
 

base/metrics/field_trial_params.cc

@@ -114,14 +114,13 @@ std::string GetFieldTrialParamValue(const std::string& trial_name,
 
 std::string GetFieldTrialParamValueByFeature(const Feature& feature,
                                              const std::string& param_name) {
-  if (!FeatureList::IsEnabled(feature))
-    return std::string();
-
-  FieldTrial* trial = FeatureList::GetFieldTrial(feature);
-  if (!trial)
+  FieldTrialParams params;
+  if (GetFieldTrialParamsByFeature(feature, &params)) {
+    auto it = params.find(param_name);
+    if (it != params.end())
+      return it->second;
+  }
   return std::string();
-
-  return GetFieldTrialParamValue(trial->trial_name(), param_name);
 }
 
 int GetFieldTrialParamByFeatureAsInt(const Feature& feature,