Add scoped class for overriding field trials.

webrtc/modules/remote_bitrate_estimator/overuse_detector_unittest.cc

Index: webrtc/modules/remote_bitrate_estimator/overuse_detector_unittest.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/overuse_detector_unittest.cc b/webrtc/modules/remote_bitrate_estimator/overuse_detector_unittest.cc
index 320cb43ccded8bcdce9c5bd16be5f92ea816e4c3..1bb28c8d9529a091faa69dfdce0144cdefcec3e4 100644
--- a/webrtc/modules/remote_bitrate_estimator/overuse_detector_unittest.cc
+++ b/webrtc/modules/remote_bitrate_estimator/overuse_detector_unittest.cc
@@ -603,14 +603,17 @@ TEST_F(OveruseDetectorTest, HighGaussianVarianceFastDrift2000Kbit30fps) {
 }
 
 class OveruseDetectorExperimentTest : public OveruseDetectorTest {
+ public:
+  OveruseDetectorExperimentTest()
+      : override_field_trials_(
+            "WebRTC-AdaptiveBweThreshold/Enabled-0.01,0.00018/") {}
+
  protected:
   void SetUp() override {
-    test::InitFieldTrialsFromString(
-        "WebRTC-AdaptiveBweThreshold/Enabled-0.01,0.00018/");
     overuse_detector_.reset(new OveruseDetector(options_));
   }
 
-  void TearDown() override { test::InitFieldTrialsFromString(""); }
+  test::ScopedFieldTrials override_field_trials_;
 };
 
 TEST_F(OveruseDetectorExperimentTest, ThresholdAdapts) {