Remove all occurrences of "using std::string".

webrtc/modules/audio_coding/neteq/test/neteq_opus_quality_test.cc

 /*
  *  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.
  */
 
 #include "webrtc/modules/audio_coding/codecs/opus/opus_interface.h"
 #include "webrtc/modules/audio_coding/codecs/opus/opus_inst.h"
 #include "webrtc/modules/audio_coding/neteq/tools/neteq_quality_test.h"
 
 using google::RegisterFlagValidator;
 using google::ParseCommandLineFlags;
-using std::string;
 using testing::InitGoogleTest;
 
 namespace webrtc {
 namespace test {
 namespace {
 
 static const int kOpusBlockDurationMs = 20;
 static const int kOpusSamplingKhz = 48;
 
 // Define switch for bit rate.
@@ skipping 178 matching lines @@
   EXPECT_GE(value, 0);
   return value;
 }
 
 TEST_F(NetEqOpusQualityTest, Test) {
   Simulate();
 }
 
 }  // namespace test
 }  // namespace webrtc