Getting rid of OutputDevice interface, moving SwitchOutputDevice() to AudioRendererSink and MediaStreamAudioRenderer interfaces.

1) OutputDevice interface eliminated;
2) OutputDeviceInfo data structure introduced; besides output parameters and device status, it provides device ID information now (at this stage it's useful for unit tests only, but it will also be used for session ID -> device ID mapping in the upcoming CL).
3) AudioRendererSink::GetOutputDevice() is replaced with AudioRendererSink::GetOutputDeviceInfo(), which returns a snapshot of output device information.
3) SwitchOutputDevice() method added to  MediaStreamAudioRenderer;
4) SwitchOutputDevice() method added to AudioRendererSink;
5) Mocks and unit tests cleaned up; sink_for_testing_ is removed from AudioRendererMixerManager, mock AudioDeviceFactory is used instead to instantiate sinks for its testing;
6) AudioRendererMixer is dependent on AudioOutputDevice no more; from now on AudioOutputDevice is visible to upper layers as AudioRendererSink only;
7) AudioOutputDevice is the only sink which implements GetOutputDeviceInfo(), all the other sinks redirect the call to their bottom layer.

See bug description for mode details and a link to doc with pictures (a bit outdated since Patchset 5 which eliminated OutputDevice interface).

BUG=595727, 506507

Review URL: https://codereview.chromium.org/1809093003

Cr-Commit-Position: refs/heads/master@{#384550}