WebRTCCertificateGenerator should not use std::string.

content/renderer/media/rtc_certificate.cc

Index: content/renderer/media/rtc_certificate.cc
diff --git a/content/renderer/media/rtc_certificate.cc b/content/renderer/media/rtc_certificate.cc
index 6345ac537cc509d894d45c209aa19728925881ae..16a57a7545f8a5c20cef3223eb7c513a598f4dd7 100644
--- a/content/renderer/media/rtc_certificate.cc
+++ b/content/renderer/media/rtc_certificate.cc
@@ -28,7 +28,9 @@ uint64_t RTCCertificate::expires() const {
 
 blink::WebRTCCertificatePEM RTCCertificate::toPEM() const {
   rtc::RTCCertificatePEM pem = certificate_->ToPEM();
-  return blink::WebRTCCertificatePEM(pem.private_key(), pem.certificate());
+  return blink::WebRTCCertificatePEM(
+      blink::WebString::fromUTF8(pem.private_key()),
+          blink::WebString::fromUTF8(pem.certificate()));
 }
 
 bool RTCCertificate::equals(const blink::WebRTCCertificate& other) const {