Add CONTENT_EXPORT for content::MediaStreamAudioLevelCalculator::Level.

content/renderer/media/media_stream_audio_level_calculator.h

Index: content/renderer/media/media_stream_audio_level_calculator.h
diff --git a/content/renderer/media/media_stream_audio_level_calculator.h b/content/renderer/media/media_stream_audio_level_calculator.h
index f093ab43251fe39b55f9898c0c9abbe0a62d3a0f..792e65c39ba7b6e844dd81cbcce332a3aafa55b8 100644
--- a/content/renderer/media/media_stream_audio_level_calculator.h
+++ b/content/renderer/media/media_stream_audio_level_calculator.h
@@ -26,7 +26,7 @@ class CONTENT_EXPORT MediaStreamAudioLevelCalculator {
   // Provides thread-safe access to the current signal level.  This object is
   // intended to be passed to modules running on other threads that poll for the
   // current signal level.
-  class Level : public base::RefCountedThreadSafe<Level> {
+  class CONTENT_EXPORT Level : public base::RefCountedThreadSafe<Level> {
    public:
     float GetCurrent() const;