Add scoped class for overriding field trials.

webrtc/test/field_trial.h

Index: webrtc/test/field_trial.h
diff --git a/webrtc/test/field_trial.h b/webrtc/test/field_trial.h
index 6503254729ac1550da7a927cca79dbf264c626fd..d448f3411d5407253a788da95d0e8938dc78fa18 100644
--- a/webrtc/test/field_trial.h
+++ b/webrtc/test/field_trial.h
@@ -12,6 +12,7 @@
 #define WEBRTC_TEST_FIELD_TRIAL_H_
 
 #include <string>
+#include <map>
 
 namespace webrtc {
 namespace test {
@@ -31,6 +32,16 @@ namespace test {
 // 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