Return nullptr from RTCCertificate::FromPEM on failure.

webrtc/base/rtccertificate.h

Index: webrtc/base/rtccertificate.h
diff --git a/webrtc/base/rtccertificate.h b/webrtc/base/rtccertificate.h
index 46d6fd427cf0e09d63e811c91dd22ab12de9c1e1..24170208eb7d4511b5d4579d217e0b73fb329a01 100644
--- a/webrtc/base/rtccertificate.h
+++ b/webrtc/base/rtccertificate.h
@@ -66,6 +66,7 @@ class RTCCertificate : public RefCountInterface {
 
   // To/from PEM, a text representation of the RTCCertificate.
   RTCCertificatePEM ToPEM() const;
+  // Can return nullptr if the certificate is invalid.
   static scoped_refptr<RTCCertificate> FromPEM(const RTCCertificatePEM& pem);
   bool operator==(const RTCCertificate& certificate) const;
   bool operator!=(const RTCCertificate& certificate) const;