Expand FeatureList to support FieldTrial association.

components/variations/proto/study.proto

Index: components/variations/proto/study.proto
diff --git a/components/variations/proto/study.proto b/components/variations/proto/study.proto
index d86a36239b2b5818ce7ed40ddae34a3dc59bc77f..493bd84358de96fdc22f4d151307d89699c0f3e4 100644
--- a/components/variations/proto/study.proto
+++ b/components/variations/proto/study.proto
@@ -44,7 +44,7 @@ message Study {
 
   // An experiment within the study.
   //
-  // Next tag: 11
+  // Next tag: 12
   message Experiment {
     // A named parameter value for this experiment.
     //
@@ -79,6 +79,30 @@ message Study {
     // Optional id used to uniquely identify this experiment for Chrome Sync.
     optional uint64 chrome_sync_experiment_id = 10;
 
+    // Specifies the feature association parameters for this experiment group.
+    //
+    // Next tag: 5
+    message FeatureAssociation {
+      // Optional list of features to enable when this experiment is selected.

[rkaplow, 2015/09/14 21:20:12]

can you mention here the relative priorities of enable/disable

[Alexei Svitkine (slow), 2015/09/14 22:03:44]

Instead, I expanded the comments to disallow the case where a feature appears in
both lists. Also mention that this does not take precedence over command-line
overrides.

+      repeated string enable_feature = 1;
+
+      // Optional list of features to disable when this experiment is selected.
+      repeated string disable_feature = 2;
+
+      // Similar to |forcing_flag|, this is an optional name of a feature which
+      // will cause this experiment to be forced on, if that feature is enabled
+      // from the command-line. Experiment with this set are not eligible for
+      // selection via a random dice roll.
+      optional string forcing_feature_on = 3;
+
+      // Similar to |forcing_flag|, this is an optional name of a feature which
+      // will cause this experiment to be forced on, if that feature is disabled

[rkaplow, 2015/09/14 21:20:12]

this is meant to be off, right? 

[Alexei Svitkine (slow), 2015/09/14 22:03:43]

Done.

+      // from the command-line. Experiment with this set are not eligible for
+      // selection via a random dice roll.
+      optional string forcing_feature_off = 4;
+    }
+    optional FeatureAssociation feature_association = 11;
+
     // Optional name of a Chrome flag that, when present, causes this experiment
     // to be forced. If the forcing_flag field is set, users will not be
     // assigned to this experiment unless that flag is present in Chrome's