Chrome is probably using a subset that represents 2% of FFMpeg. I'd guess they have thoroughly audited the code paths that are actually used to decode about 10 media different that chrome supports.

For instance, one fix mentioned was a fix the demuxer for .4xm files. 4xm is a proprietary container format for a now defunct company. Chrome would never have to worry about this bug, because it doesn't need to support playback of .4xm files.