Reland of: media::VpxVideoDecoder cleanup

Relanding http://crrev.com/1438233003, which got reverted due
to screwing with the MemoryPool lifetime, causing a use-after-free.

This CL lands the original CL, keeping MemoryPool as ref-counted
and adding some comments on it.

Original CL description -----------------------------------------------
Couple of deep-cut cleanups:

- Inner class MemoryPool is ref-counted thread safe when it
doesn't need to, removed it, using WeakPtr for the only Bind
use case, which anyway already had a BindToCurrentLoop().
- moved MemoryPool's registration & dereg with
MemoryDumpManager to its ctor/dtor.

- reshuffled a few method to follow declaration order (sorry
that it makes review harder!).
- removed VpxVideoDecoder's |task_runner_|, which is used
mostly for thread checking (with a note in Reset()).
- merged Decode() and DecodeBuffer() following a TODO
suggestion. This makes |decode_cb_| superfluous, removed.
- added vpx error stringification (useful!).
- clarified in DCHECK()s the supported pixel formats vs codec
and other assumptions that were not so evident to me.
- using libyuv::I420 copy ISO Copy{Y,U,V}Plane(). Also,
MakeOpaqueAPlane() > libyuv::SetPlane() and
CopyAPlane() -> libyuv::CopyPlane().
- changed some if-if to early returns.

Note that VP9 could have supported Alpha channel if
looking at the code in ConfigureDecoder(), but not if
we looked at CopyVpxImageTo(). Made this evident.

BUG=554196, 554737
TEST=Tried bags of tests including this
functionality, in particular
./out/Debug/media_unittests --gtest_filter="PipelineIntegrationTest.*_VP*"
and anything relating to playing a WebM file
from Chrome (big buck bunny).

Review URL: https://codereview.chromium.org/1447343004

Cr-Commit-Position: refs/heads/master@{#360665}

1 files changed, 1 insertions, 1 deletions

diff --git a/media/renderers/default_renderer_factory.cc b/media/renderers/default_renderer_factory.cc
index 6ebc372..c1c2aaa 100644
--- a/media/renderers/default_renderer_factory.cc
+++ b/media/renderers/default_renderer_factory.cc
@@ -78,7 +78,7 @@ scoped_ptr<Renderer> DefaultRendererFactory::CreateRenderer(
     video_decoders.push_back(new GpuVideoDecoder(gpu_factories_));
 
 #if !defined(MEDIA_DISABLE_LIBVPX)
-  video_decoders.push_back(new VpxVideoDecoder(media_task_runner));
+  video_decoders.push_back(new VpxVideoDecoder());
 #endif
 
 #if !defined(MEDIA_DISABLE_FFMPEG) && !defined(DISABLE_FFMPEG_VIDEO_DECODERS)