Cast: Fix audio pipeline

1. Add audio decoded callback  - guarantees that we will return an audio frame when invoked.
2. Bug fixes in sender.cc
3. Add a return in transport.cc when we drop a packet.

BUG=323703

Review URL: https://codereview.chromium.org/102293004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238992 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238992