Improve and simplify AudioOutputDispatcher.

AudioOutputDispatcher does a number of things which no longer make
sense in a world where we don't create tons of physical streams:

    - Streams are not immediately recycled but instead inserted
    into a paused queue.  This was done to improve cycle time
    when physical streams were rapidly paused and played.

    - If any stream was paused previously and then closed, every
    new stream opened will open two streams.  Again this was to
    improve cycle time for paused and played streams.

For HTML5, this has been handled by AudioRendererMixer for some time.
WebRTC, WebAudio, and PPAPI generally don't start and stop streams
repeatedly in a way that this code is beneficial.

As such, all of that code has been removed.  As a hedge, one stream
is left open after Close() until the close timer fires.  This allows
cycle time to be improved in cases of interleaved Start/Stop cycles.

I've additionally cleaned up the code to use std::algorithm where
applicable, removed unnecessary WeakPtr setup, and cleaned up the
unittests.

BUG=none
TEST=audio plays, unittests pass.

Review URL: https://codereview.chromium.org/27605002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230334 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230334