Bug 551277 adds the Chromium code and integrates it with the new video layer painting code. The Chromium code uses a C fallback if MMX is not available but this is done at compile time. I modified it do runtime detection using Mozilla's SSE.h and moved the generic C routine from the Linux specific implementation to be used on all platforms that don't have MMX. Some other minor modifications were done (function name, namspacing).
I added an 'update.sh' script that can be used to update against the latest Chromium code. Given the path to the Chromium source it applies a patch for the changes I made.
Bug 551378 adds YCbCr 4:4:4 conversion (YV24). The Chromium code supported 4:2:0 and 4:2:2 (YV12 and YV16 respectively). Theora also allows 4:4:4 per spec. There aren't many 4:4:4 Theora videos out in the wild and the fix in this bug only adds support in the slower C implementation. I detect if the video is a 4:4:4 video and fallback to the C code.
At some point optimized MMX routines should be done for this. I've raised Bug 551844 for this. This would probably make a good first bug for anyone who wants something small and self contained to work on. It would be good to contribute this back to Chromium when done too.
Xiph has a test suite with example videos in the different formats.