Make RTCCertificate::identity_ const

Bug: none
Change-Id: Id66268a7b23704b1526c698901e4875fbfc13eb2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/213184
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33588}

rtc_base/rtc_certificate.h

@@ -87,7 +87,7 @@ class RTC_EXPORT RTCCertificate : public RefCountInterface {
  private:
   // The SSLIdentity is the owner of the SSLCertificate. To protect our
   // GetSSLCertificate() we take ownership of |identity_|.
-  std::unique_ptr<SSLIdentity> identity_;
+  const std::unique_ptr<SSLIdentity> identity_;
 };
 
 }  // namespace rtc