This class is in charge of creating media::Renderer to be used by the meida
pipeline.

Benefits of this CL:
- We have a central place for media::Renderer creation.
- WebMediaPlayerImpl is much cleaner.
- Creation of MojoRendererImpl is delayed from construction time to load time.

BUG=416570
TEST=Existing tests pass.

Review URL: https://codereview.chromium.org/783003002

Cr-Commit-Position: refs/heads/master@{#307808}