Add simpler API for changing parameters in tests.

When in need of changing variation parameters in unittests, one has to
dig unneccessarily deep into the inner workings of variations::.

This CL adds a simple class to use in unittests.
Simplifying existing unittest code is a matter of further CLs.

BUG=629013

Review-Url: https://codereview.chromium.org/2145963002
Cr-Commit-Position: refs/heads/master@{#405992}

components/variations/variations_associated_data.cc

@@ -16,6 +16,8 @@ namespace variations {
 
 namespace {
 
+const char kGroupTesting[] = "Testing";
+
 // The internal singleton accessor for the map, used to keep it thread-safe.
 class GroupMapAccessor {
  public:
@@ -252,6 +254,26 @@ std::string GetVariationParamValueByFeature(const base::Feature& feature,
 // They simply wrap existing functions in this file.
 namespace testing {
 
+VariationParamsManager::VariationParamsManager(
+    const std::string& trial_name,
+    const std::map<std::string, std::string>& params) {
+  SetVariationParams(trial_name, params);
+}
+
+VariationParamsManager::~VariationParamsManager() {
+  ClearAllVariationIDs();
+  ClearAllVariationParams();
+  field_trial_list_.reset();
+}
+
+void VariationParamsManager::SetVariationParams(
+    const std::string& trial_name,
+    const std::map<std::string, std::string>& params) {
+  field_trial_list_.reset(new base::FieldTrialList(nullptr));
+  variations::AssociateVariationParams(trial_name, kGroupTesting, params);
+  base::FieldTrialList::CreateFieldTrial(trial_name, kGroupTesting);
+}
+
 void ClearAllVariationIDs() {
   GroupMapAccessor::GetInstance()->ClearAllMapsForTesting();
 }

components/variations/variations_associated_data.h

@@ -164,6 +164,31 @@ 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
+// are used by other members upon construction). You can change them later
+// arbitrarily many times using the SetVariationParams function. Internally, it
+// creates a FieldTrialList as a member. It works well for multiple tests of a
+// given test class, as it clears the parameters when this class is destructed.
+// Note that it clears all parameters (not just those registered here).
+class VariationParamsManager {
+ public:
+  VariationParamsManager(const std::string& trial_name,
+                         const std::map<std::string, std::string>& params);
+  ~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_;
+
+  DISALLOW_COPY_AND_ASSIGN(VariationParamsManager);
+};
+
 // Clears all of the mapped associations.
 void ClearAllVariationIDs();