media: Convert Once{Callback/Closure} work in Demuxer/Renderer.

Converted Demuxer::Seek to use OnceCallback and Renderer::Initialize
to use OnceClosure. These are landing in the same large CL because
media::SerialRunner is used for the callbacks for each of these
calls and needed to switch from repeating to once at the same time.

Bug: 1007799, 1007810, 751838
Change-Id: Ib0396c4db5d1cba47efbe1774d49c0217439943d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879558
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Chris Mumford <cmumford@google.com>
Cr-Commit-Position: refs/heads/master@{#709690}

diff --git a/media/renderers/renderer_impl.h b/media/renderers/renderer_impl.h
index 5b1e0664..16a4c36 100644
--- a/media/renderers/renderer_impl.h
+++ b/media/renderers/renderer_impl.h

@@ -55,7 +55,7 @@
   // Renderer implementation.
   void Initialize(MediaResource* media_resource,
                   RendererClient* client,
-                  const PipelineStatusCB& init_cb) final;
+                  PipelineStatusCallback init_cb) final;
   void SetCdm(CdmContext* cdm_context,
               const CdmAttachedCB& cdm_attached_cb) final;
   void Flush(base::OnceClosure flush_cb) final;
@@ -208,7 +208,7 @@
   RendererClient* client_;
 
   // Temporary callback used for Initialize() and Flush().
-  PipelineStatusCB init_cb_;
+  PipelineStatusCallback init_cb_;
   base::OnceClosure flush_cb_;
 
   std::unique_ptr<RendererClientInternal> audio_renderer_client_;