WebRTCCertificateGenerator should not use std::string.

content/renderer/media/rtc_certificate_generator.cc

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/media/rtc_certificate_generator.h"
 
 #include <string>
 #include <utility>
 
 #include "base/macros.h"
@@ skipping 133 matching lines @@
   observer->onError();
 #endif
 }
 
 bool RTCCertificateGenerator::isSupportedKeyParams(
     const blink::WebRTCKeyParams& key_params) {
   return WebRTCKeyParamsToKeyParams(key_params).IsValid();
 }
 
 std::unique_ptr<blink::WebRTCCertificate> RTCCertificateGenerator::fromPEM(
-    const std::string& pem_private_key,
-    const std::string& pem_certificate) {
+    blink::WebString pem_private_key,
+    blink::WebString pem_certificate) {
   rtc::scoped_refptr<rtc::RTCCertificate> certificate =
       rtc::RTCCertificate::FromPEM(
-          rtc::RTCCertificatePEM(pem_private_key, pem_certificate));
+          rtc::RTCCertificatePEM(pem_private_key.utf8(),
+              pem_certificate.utf8()));
   return std::unique_ptr<blink::WebRTCCertificate>(
       new RTCCertificate(certificate));
 }
 
 }  // namespace content