[Chromecast] Remove weak symbols in external_audio_pipeline_shlib.h.

Bug: internal b/127656475

Change-Id: Ib7681e6de5693ab25c5df96274b691b2ad0aa9a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530838Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Steven Zhu <jz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642392}

chromecast/public/media/external_audio_pipeline_shlib.h

@@ -128,12 +128,12 @@ class CHROMECAST_EXPORT ExternalAudioPipelineShlib {
 
   // Adds an external media metadata observer.
   static void AddExternalMediaMetadataChangeObserver(
-      ExternalMediaMetadataChangeObserver* observer) __attribute__((__weak__));
+      ExternalMediaMetadataChangeObserver* observer);
 
   // Removes an external media volume observer. After this is called, the
   // implementation must not call any more methods on the observer.
   static void RemoveExternalMediaMetadataChangeObserver(
-      ExternalMediaMetadataChangeObserver* observer) __attribute__((__weak__));
+      ExternalMediaMetadataChangeObserver* observer);
 
   // Returns an instance of MixerOutputStream from the shared library.
   // Caller will take ownership of the returned pointer.