Allow trials to associate without overriding a feature.

This adds a heuristic such that if a study only references a single
feature from any of its groups, its other groups will be associated
with that feature as well, for reporting only. This allows a config like
25% Enabled, 25% Disabled and 50% Default to still report the users
in the default group to UMA, so that they show up in the data.

More details in the code comments.

BUG=587135

Review URL: https://codereview.chromium.org/1809633003

Cr-Commit-Position: refs/heads/master@{#381737}

3 files changed, 37 insertions, 1 deletions

diff --git a/base/feature_list.cc b/base/feature_list.cc
index d10c60b..089da64 100644
--- a/base/feature_list.cc
+++ b/base/feature_list.cc
@@ -108,6 +108,8 @@ void FeatureList::GetFeatureOverrides(std::string* enable_overrides,
       case OVERRIDE_DISABLE_FEATURE:
         target_list = disable_overrides;
         break;
+      case OVERRIDE_USE_DEFAULT:
+        continue;
     }
 
     if (!target_list->empty())
@@ -177,7 +179,10 @@ bool FeatureList::IsFeatureEnabled(const Feature& feature) {
       entry.field_trial->group();
 
     // TODO(asvitkine) Expand this section as more support is added.
-    return entry.overridden_state == OVERRIDE_ENABLE_FEATURE;
+
+    // If marked as OVERRIDE_USE_DEFAULT, simply return the default state below.
+    if (entry.overridden_state != OVERRIDE_USE_DEFAULT)
+      return entry.overridden_state == OVERRIDE_ENABLE_FEATURE;
   }
   // Otherwise, return the default state.
   return feature.default_state == FEATURE_ENABLED_BY_DEFAULT;
diff --git a/base/feature_list.h b/base/feature_list.h
index 875d3b5..90128cf 100644
--- a/base/feature_list.h
+++ b/base/feature_list.h
@@ -91,6 +91,7 @@ class BASE_EXPORT FeatureList {
 
   // Specifies whether a feature override enables or disables the feature.
   enum OverrideState {
+    OVERRIDE_USE_DEFAULT,
     OVERRIDE_DISABLE_FEATURE,
     OVERRIDE_ENABLE_FEATURE,
   };
diff --git a/base/feature_list_unittest.cc b/base/feature_list_unittest.cc
index 11cf179..7155e86 100644
--- a/base/feature_list_unittest.cc
+++ b/base/feature_list_unittest.cc
@@ -6,6 +6,7 @@
 
 #include <stddef.h>
 
+#include <algorithm>
 #include <utility>
 
 #include "base/format_macros.h"
@@ -178,6 +179,35 @@ TEST_F(FeatureListTest, FieldTrialOverrides) {
   }
 }
 
+TEST_F(FeatureListTest, FieldTrialAssociateUseDefault) {
+  FieldTrialList field_trial_list(nullptr);
+  scoped_ptr<FeatureList> feature_list(new FeatureList);
+
+  FieldTrial* trial1 = FieldTrialList::CreateFieldTrial("TrialExample1", "A");
+  FieldTrial* trial2 = FieldTrialList::CreateFieldTrial("TrialExample2", "B");
+  feature_list->RegisterFieldTrialOverride(
+      kFeatureOnByDefaultName, FeatureList::OVERRIDE_USE_DEFAULT, trial1);
+  feature_list->RegisterFieldTrialOverride(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_USE_DEFAULT, trial2);
+  RegisterFeatureListInstance(std::move(feature_list));
+
+  // Initially, neither trial should be active.
+  EXPECT_FALSE(FieldTrialList::IsTrialActive(trial1->trial_name()));
+  EXPECT_FALSE(FieldTrialList::IsTrialActive(trial2->trial_name()));
+
+  // Check the feature enabled state is its default.
+  EXPECT_TRUE(FeatureList::IsEnabled(kFeatureOnByDefault));
+  // The above should have activated |trial1|.
+  EXPECT_TRUE(FieldTrialList::IsTrialActive(trial1->trial_name()));
+  EXPECT_FALSE(FieldTrialList::IsTrialActive(trial2->trial_name()));
+
+  // Check the feature enabled state is its default.
+  EXPECT_FALSE(FeatureList::IsEnabled(kFeatureOffByDefault));
+  // The above should have activated |trial2|.
+  EXPECT_TRUE(FieldTrialList::IsTrialActive(trial1->trial_name()));
+  EXPECT_TRUE(FieldTrialList::IsTrialActive(trial2->trial_name()));
+}
+
 TEST_F(FeatureListTest, CommandLineTakesPrecedenceOverFieldTrial) {
   ClearFeatureListInstance();