Use base::nullopt instead of std::string() for Optional parameter in...

Use base::nullopt instead of std::string() for Optional parameter in WebrtcAudioPrivateGetAssociatedSinkFunction.

DCHECK was hit because of incorrect usage; fixed with this change.

Bug: 804836
Change-Id: Ia09f9a6bc563b6468236fd821ade49e1cb8f0fa7
Reviewed-on: https://chromium-review.googlesource.com/883343Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Henrik Grunell <grunell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531685}

chrome/browser/extensions/api/webrtc_audio_private/webrtc_audio_private_api.cc

@@ -269,7 +269,7 @@ void WebrtcAudioPrivateGetAssociatedSinkFunction::
     }
   }
   if (raw_source_id.empty()) {
-    CalculateHMACOnIOThread(std::string());
+    CalculateHMACOnIOThread(base::nullopt);
     return;
   }
   GetAudioSystem()->GetAssociatedOutputDeviceID(