Adding support for the new media_input_source_description field of media session.

New string field input_source_description would contain a description of
current media input source. For example, if some device has two AUX inputs,
this filed could contain 'Left AUX'.

Bug: internal b/118644722

Change-Id: I8a75f9663d8a4b0553eaa0ed177c7fc1af1bef4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880274Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Commit-Queue: Luke Halliwell (slow) <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732236}

chromecast/public/media/external_audio_pipeline_shlib.h

@@ -68,6 +68,7 @@ class CHROMECAST_EXPORT ExternalAudioPipelineShlib {
     std::string genre;
     int track_num = -1;
     MediaInputSource source = MediaInputSource::HDMI;
+    std::string source_description;
     MediaPlaybackState state = MediaPlaybackState::UNKNOWN_STATE;
   };