Include missing base/callback.h

Bug: 1031540
Change-Id: I7a64d749c2c1f7484aedb8f7a7f74088972400f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270661Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Byungchul Kim <byungchul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783070}

chromecast/media/api/cast_sounds_manager.h

@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "base/callback.h"
 #include "base/time/time.h"
 
 namespace chromecast {