Allow overriding variation parameter via chrome://flags.

components/flags_ui/feature_entry.h

Index: components/flags_ui/feature_entry.h
diff --git a/components/flags_ui/feature_entry.h b/components/flags_ui/feature_entry.h
index 4f7f0819a489962e0a195b4b79fd0c8ae9910e0f..8e706f44d93c252444465625b982338132b1192f 100644
--- a/components/flags_ui/feature_entry.h
+++ b/components/flags_ui/feature_entry.h
@@ -47,6 +47,23 @@ struct FeatureEntry {
     // have three states: Default, Enabled, Disabled. When not specified or set
     // to Default, the normal default value of the feature is used.
     FEATURE_VALUE,
+
+    // Corresponds to a base::Feature + a fixed list of variations of parameters
+    // of the given feature [V_1, ..., V_n]. The entry will have n+2 states:

[Alexei Svitkine (slow), 2016/06/10 15:46:49]

I find "a fixed list of variations of parameters of a given feature" a little
vague about the exact structure. Maybe something like "Corresponds to a
base::Feature and additional options that specify variation parameters." Then,
you can describe that each option can specify a set of variation params.

[jkrcal, 2016/06/14 10:04:22]

Done.

+    // Default, Enabled: V_1, ..., Enabled: V_n, Disabled. When not specified or
+    // set to Default, the normal default values of the feature and of the
+    // parameters are used.
+    FEATURE_WITH_VARIATIONS_VALUE,
+  };
+
+  // Describes state of a feature.
+  enum State {

[Alexei Svitkine (slow), 2016/06/10 15:46:49]

Nit: FeatureState? assuming this only applies to features and not other cases.

[jkrcal, 2016/06/14 10:04:22]

Done.

+    // The state of the feature is not overridden by the user.
+    DEFAULT,
+    // The feature is enabled by the user.
+    ENABLED,
+    // The feature is disabled by the user.
+    DISABLED,
   };
 
   // Used for MULTI_VALUE types to describe one of the possible values the user
@@ -61,6 +78,20 @@ struct FeatureEntry {
     const char* command_line_value;
   };
 
+  // Configures one parameter for FEATURE_WITH_VARIATIONS_VALUE.
+  struct FeatureParam {
+    const char* param_name;
+    const char* param_value;
+  };
+
+  // Specified one variation (list of parameter values) for
+  // FEATURE_WITH_VARIATIONS_VALUE.
+  struct FeatureVariation {
+    const char* description_text;

[Alexei Svitkine (slow), 2016/06/10 15:46:49]

Add a comment mentioning how it's used.

Also, explain why this doesn't need to be a description_id - that is, because
chrome://flags text should not be translated. Also mention that the other parts
use ids for historical reasons and can realistically also be moved to this.

[jkrcal, 2016/06/14 10:04:22]

Done.

+    const FeatureParam* params;
+    int num_params;
+  };
+
   // The internal name of the feature entry. This is never shown to the user.
   // It _is_ however stored in the prefs file, so you shouldn't change the
   // name of existing flags.
@@ -99,15 +130,39 @@ struct FeatureEntry {
   // This is used if type is MULTI_VALUE.
   const Choice* choices;
 
-  // Number of |choices|.
-  // This is used if type is MULTI_VALUE.
-  int num_choices;
+  // This is used if type is FEATURE_WITH_VARIATIONS_VALUE. The first variation
+  // is the default "Enabled" variation, its description_id is disregarded.
+  const FeatureVariation* feature_variations;
+
+  // The name of the FieldTrial in which the selected variation parameters
+  // should be registered. This is used if type is
+  // FEATURE_WITH_VARIATIONS_VALUE.
+  const char* feature_trial_name;
+
+  // Number of options to choose from. This is used if type is MULTI_VALUE,
+  // ENABLE_DISABLE_VALUE, FEATURE_VALUE, or FEATURE_WITH_VARIATIONS_VALUE.
+  int num_options;

[Alexei Svitkine (slow), 2016/06/10 15:46:49]

Nit: Keep this above the two fields above, since it applies more generally than
just to features.

[jkrcal, 2016/06/14 10:04:22]

I moved it even above the choices, I think it is clearer this way.

+
+  // Returns the name used in prefs for the option at the specified |index|.
+  // Only used for types that use |num_options|.
+  std::string NameForOption(int index) const;
+
+  // Returns the human readable description for the option at |index|.
+  // Only used for types that use |num_options|.
+  base::string16 DescriptionForOption(int index) const;
+
+  // Returns the choice for the option at |index|. Only applicable for types

[Alexei Svitkine (slow), 2016/06/10 15:46:49]

types -> type

[jkrcal, 2016/06/14 10:04:22]

Done.

+  // FEATURE_MULTI.

[Alexei Svitkine (slow), 2016/06/10 15:46:49]

FEATURE_WITH_VARIATIONS_VALUE?

[jkrcal, 2016/06/14 10:04:22]

Addressed by VariationForOption(). Maybe I don't understand :)

+  const FeatureEntry::Choice& ChoiceForOption(int index) const;
 
-  // Returns the name used in prefs for the choice at the specified |index|.
-  std::string NameForChoice(int index) const;
+  // Returns the state of the feature at |index|. Only applicable for types
+  // FEATURE_VALUE and FEATURE_WITH_VARIATIONS_VALUE.
+  FeatureEntry::State StateForOption(int index) const;
 
-  // Returns the human readable description for the choice at |index|.
-  base::string16 DescriptionForChoice(int index) const;
+  // Returns the variation for the option at |index| or nullptr if there is no
+  // variation associated at |index|. Only applicable for types FEATURE_VALUE
+  // and FEATURE_WITH_VARIATIONS_VALUE.
+  const FeatureEntry::FeatureVariation* VariationForOption(int index) const;
 };
 
 namespace testing {