Avoid dereferencing null pointer if generateCertificate fails.

Bug: chromium:820806
Change-Id: I36340c31ee6c5c88a4049d346ed9af9dbff43392
Reviewed-on: https://chromium-review.googlesource.com/959525Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542812}

content/renderer/media/webrtc/rtc_certificate_generator.cc

@@ -93,7 +93,9 @@ class RTCCertificateGeneratorRequest
         FROM_HERE,
         base::BindOnce(&RTCCertificateGeneratorRequest::DoCallbackOnMainThread,
                        this, std::move(observer),
-                       std::make_unique<RTCCertificate>(certificate)));
+                       certificate
+                           ? std::make_unique<RTCCertificate>(certificate)
+                           : nullptr));
   }
 
   void DoCallbackOnMainThread(