Add return after NOT_REACHED() in eventlog unittest.

Bug: webrtc:9457
Change-Id: If4728d05d832f72871c25ddce93a72be5089be40
Reviewed-on: https://webrtc-review.googlesource.com/86122
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23763}

logging/rtc_event_log/rtc_event_log_unittest_helper.cc

@@ -235,6 +235,8 @@ EventGenerator::NewRtcpPacketIncoming() {
     }
     default:
       RTC_NOTREACHED();
+      rtc::Buffer buffer;
+      return rtc::MakeUnique<RtcEventRtcpPacketIncoming>(buffer);
   }
 }
 
@@ -254,6 +256,8 @@ EventGenerator::NewRtcpPacketOutgoing() {
     }
     default:
       RTC_NOTREACHED();
+      rtc::Buffer buffer;
+      return rtc::MakeUnique<RtcEventRtcpPacketOutgoing>(buffer);
   }
 }