Add scoped class for overriding field trials.

webrtc/test/field_trial.h

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #ifndef WEBRTC_TEST_FIELD_TRIAL_H_
 #define WEBRTC_TEST_FIELD_TRIAL_H_
 
 #include <string>
+#include <map>
 
 namespace webrtc {
 namespace test {
 
 // Parses enabled field trials from a string config, such as the one passed
 // to chrome's argument --force-fieldtrials and initializes webrtc::field_trial
 // with such a config.
 //  E.g.:
 //    "WebRTC-experimentFoo/Enabled/WebRTC-experimentBar/Enabled100kbps/"
 //    Assigns the process to group "Enabled" on WebRTCExperimentFoo trial
 //    and to group "Enabled100kbps" on WebRTCExperimentBar.
 //
 //  E.g. invalid config:
 //    "WebRTC-experiment1/Enabled"  (note missing / separator at the end).
 //
 // Note: This method crashes with an error message if an invalid config is
 // passed to it. That can be used to find out if a binary is parsing the flags.
 void InitFieldTrialsFromString(const std::string& config);
 
+// This class is used to override field-trial configs within specific tests.
+// After this class goes out of scope previous field trials will be restored.
+class ScopedFieldTrials {
+ public:
+  explicit ScopedFieldTrials(const std::string& config);
+  ~ScopedFieldTrials();
+ private:
+  const std::map<std::string, std::string> previous_field_trials_;
+};
+
 }  // namespace test
 }  // namespace webrtc
 
 #endif  // WEBRTC_TEST_FIELD_TRIAL_H_