renderer
1. Added 4 IPC messages and corresponding handlers for
audio:
  - RequestAudioPacket(int stream_id)
    Browser process is hungry for audio packet, notify
    renderer process to provide more.

  - NotifyAudioStreamCreated(int stream_id, SharedMemoryHandler
buffer, int len)
    Notify stream created event and provide buffer for
    filling in the future.

  - NotifyAudioStreamStateChanged(int stream_id, enum state,
nt info)
    The internal state of the audio stream has chagned,
notify renderer
    process of the change. int info provides additional
information of the
    change, e.g. platform specific error code.

  - NotifyAudioStreamVolume(int stream_id, double left, double right)
    Notify the current volume for the audio stream.
2. Added methods to RenderView for creating audio streams
and delegate audio related requests to browser process with
IPC. Now the registration and bookkeeping of
AudioRendererImpl happens in RenderView (see
audio_renderers_). The reason being that the code is almost
just an base::IDMap that doesn't worth creating a new class.

Review URL: http://codereview.chromium.org/20410

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@10078 0039d316-1c4b-4281-b951-d872f2087c98