Make delay stat optional.

The delay_ms stat in AudioprocessStats should be an Optional, because its value is not always computed. This CL changes it to an optional.

Bug: webrtc:8569
Change-Id: I42fd7a86b975c766b685444bf1829511f790da2a
Reviewed-on: https://webrtc-review.googlesource.com/33320
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21293}

modules/audio_processing/include/audio_processing_statistics.h

@@ -48,7 +48,7 @@ struct AudioProcessingStats {
   // The instantaneous delay estimate produced in the AEC. The unit is in
   // milliseconds and the value is the instantaneous value at the time of the
   // call to |GetStatistics()|.
-  int delay_ms;
+  rtc::Optional<int32_t> delay_ms;
 };
 
 }  // namespace webrtc