Revert of Revert of Implement software fallback for PPB_VideoDecoder.

Revert of Revert of Implement software fallback for PPB_VideoDecoder. (https://codereview.chromium.org/337683002/)

Reason for revert:
Revert of revert, which apparently wasn't a full revert.

Original issue's description:
> Revert of Implement software fallback for PPB_VideoDecoder. (https://codereview.chromium.org/311853005/)
> 
> Reason for revert:
> Broke blink Linux tests compile.http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests/builds/37259
> 
> Original issue's description:
> > Implement software fallback for PPB_VideoDecoder.
> > This modifies the proxy to implement software fallback mode.
> > The main change is to the host, which now can work with
> > media::VideoDecoders.
> > 
> > media::VideoDecoder works differently from media::VideoDecodeAccelerator
> > so an adapter object, content::VideoDecoderShim is defined. It lives on the main thread and drives the actual decoder on the media thread via a child DecoderImpl class, which sends back frames of video. VideoDecoderShim receives those and converts frames to GL textures.
> > 
> > gpu::Mailboxes are used so the host can create textures that are aliased
> > to the plugin's textures.
> > 
> > The test plugin has been changed to include bitstream data for VP8 in order to
> > test the software decoder. The data is in ppapi/examples/video_decode/testdata.h
> > alongside the H264 data. The file diff is too large for this site but is structured
> > something like this:
> > 
> > const unsigned char kData[] = {
> > #if defined USE_VP8_TESTDATA_INSTEAD_OF_H264
> > ...  lots of VP8 data
> > 
> > #else  // !USE_VP8_TESTDATA_INSTEAD_OF_H264
> > ...  lots of H264 data
> > 
> > #endif  // USE_VP8_TESTDATA_INSTEAD_OF_H264
> > };
> > 
> > 
> > There is a TODO to convert the example to load a file. I'm not sure how to go
> > about that but am willing to do the work if someone can point the way.
> > 
> > BUG=281689
> > R=dmichael@chromium.org, fischman@chromium.org, sievers@chromium.org, tsepez@chromium.org
> > 
> > Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277012
> 
> TBR=dmichael@chromium.org,fischman@chromium.org,igorc@chromium.org,piman@chromium.org,sievers@chromium.org,tsepez@chromium.org,bbudge@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=281689
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277015

TBR=dmichael@chromium.org,fischman@chromium.org,igorc@chromium.org,piman@chromium.org,sievers@chromium.org,tsepez@chromium.org,bbudge@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=281689
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277020

[Stephen Chennney, 2014-06-13 14:57:37]

Created Revert of Revert of Implement software fallback for PPB_VideoDecoder.

[commit-bot: I haz the power, 2014-06-13 14:59:08]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/schenney@chromium.org/333903002/1

[commit-bot: I haz the power, 2014-06-13 15:00:18]

Change committed as 277020