Currently, only the DemuxerHost interface allows the propragation of
duration changes; the demuxer is responsible for detecting duration
changes and notifying the Pipeline.

In the case of the MediaPlayerRenderer, we use a MediaUrlDemuxer, which
never receives duration updates. Instead, all metadata parsing happens
on the browser side, in the MediaPlayerRenderer. We therefore need a
way to propagate the duration changes from the MediaPlayerRenderer to
the pipeline.

This change adds an OnDurationChange method to both the
media::RendererClient and the mojom::RendererClient interfaces.

BUG=635991

Review-Url: https://codereview.chromium.org/2243683002
Cr-Commit-Position: refs/heads/master@{#411893}