Adding support for logging severity LS_NONE.

Bug: webrtc:8735
Change-Id: I07247ce67983f873febb8d8d32c25032a4608eae
Reviewed-on: https://webrtc-review.googlesource.com/c/40400
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Peter Hanspers <peterhanspers@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25197}

sdk/objc/api/logging/RTCCallbackLogger.mm

@@ -86,6 +86,8 @@ class CallbackLogSink : public rtc::LogSink {
       return rtc::LS_WARNING;
     case RTCLoggingSeverityError:
       return rtc::LS_ERROR;
+    case RTCLoggingSeverityNone:
+      return rtc::LS_NONE;
   }
 }