Add simpler API for changing parameters in tests.

components/variations/variations_associated_data.h

Index: components/variations/variations_associated_data.h
diff --git a/components/variations/variations_associated_data.h b/components/variations/variations_associated_data.h
index 3eb9567cc3afe41848602b2fc9a2eabdaa00de77..1e3833b6b4720688359bef665871f122e30f5303 100644
--- a/components/variations/variations_associated_data.h
+++ b/components/variations/variations_associated_data.h
@@ -164,6 +164,29 @@ std::string GetVariationParamValueByFeature(const base::Feature& feature,
 // Expose some functions for testing.
 namespace testing {
 
+// Use this class as a member in your test class to set variation params for
+// your tests. You can directly set the parameters in the constructor (if they

[Alexei Svitkine (slow), 2016/07/15 14:51:01]

Also mention that it creates a FieldTrialList as a member.

[jkrcal, 2016/07/18 13:09:01]

Done.

+// are used by other members upon construction). You can change them later
+// arbitrarily many times using the SetVariationParams function. The parameters
+// are automatically cleared when this class is destructed so it works well for

[Alexei Svitkine (slow), 2016/07/15 14:51:01]

Please make this sentence more precise to specify that this class will clear all
params on destruction, not just ones it registered.

[jkrcal, 2016/07/18 13:09:01]

Done.

+// multiple tests of a given test class.
+class VariationParamsManager {
+ public:
+  VariationParamsManager(const std::string& trial_name,
+                         const std::map<std::string, std::string>& params);
+

[Alexei Svitkine (slow), 2016/07/15 14:51:01]

Nit: No need for empty line here.

[jkrcal, 2016/07/18 13:09:01]

Done.

+  ~VariationParamsManager();
+
+  // Associates |params| with the given |trial_name|. It creates a new group,
+  // used only for testing. Between two calls of this function,
+  // ClearAllVariationParams() has to be called.
+  void SetVariationParams(const std::string& trial_name,
+                          const std::map<std::string, std::string>& params);
+
+ private:
+  std::unique_ptr<base::FieldTrialList> field_trial_list_;
+};

[Alexei Svitkine (slow), 2016/07/15 14:51:01]

DISALLOW_COPY_AND_ASSIGN()

[jkrcal, 2016/07/18 13:09:01]

Done.

+
 // Clears all of the mapped associations.
 void ClearAllVariationIDs();