Moved codec-specific audio packet splitting into decoders.

There's still some code run specifically for Opus w/ FEC. It will be
addressed in a separate CL.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2326003002
Cr-Commit-Position: refs/heads/master@{#14319}

webrtc/modules/audio_coding/audio_coding.gypi

@@ -186,6 +186,26 @@
         'include/audio_coding_module_typedefs.h',
       ],
     },
+    {
+      'target_name': 'legacy_encoded_audio_frame',
+      'type': 'static_library',
+      'dependencies': [
+        '<(webrtc_root)/common.gyp:webrtc_common',
+        'audio_decoder_interface',
+      ],
+      'include_dirs': [
+        '<(webrtc_root)',
+      ],
+      'direct_dependent_settings': {
+        'include_dirs': [
+          '<(webrtc_root)',
+        ],
+      },
+      'sources': [
+        'codecs/legacy_encoded_audio_frame.cc',
+        'codecs/legacy_encoded_audio_frame.h',
+      ],
+    },
   ],
   'conditions': [
     ['include_opus==1', {