[Chromecast] Add default enum values in StreamInfo struct

Even though they are enum class instead of enum, we'd give them default
value in the struct, so as to avoid them be initialized to random value.

Bug: NONE.
Test: CQ.
Change-Id: If0bd613b2eb6afa10bbc0bb8d5904c9e92c3ba00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490856
Auto-Submit: Kehuang Li <kehuangli@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Kehuang Li <kehuangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820427}

chromecast/media/audio/capture_service/constants.h

@@ -70,10 +70,10 @@ enum class MessageType : uint8_t {
 };
 
 struct StreamInfo {
-  StreamType stream_type;
-  AudioCodec audio_codec;
+  StreamType stream_type = StreamType::kMicRaw;
+  AudioCodec audio_codec = AudioCodec::kPcm;
   int num_channels = 0;
-  SampleFormat sample_format;
+  SampleFormat sample_format = SampleFormat::INTERLEAVED_INT16;
   int sample_rate = 0;
   int frames_per_buffer = 0;
 };