WMPI owns an instance of PipelineImpl and passes its address to
a PipelineController. WMPI calls method on both |pipeline_| and
|pipeline_controller_|.

This CL moves the ownership of PipelineImpl to the PipelineController,
and exposes the rest of the Pipeline interface through the
PipelineController. This will simplify reasoning about Pipeline
callbacks and states changes by forcing all calls from WMPI to go
through |pipeline_controller_|.

bug=

Review-Url: https://codereview.chromium.org/2689863002
Cr-Commit-Position: refs/heads/master@{#452764}