While looking through the shutdown hangs, I've noticed an oddly large number of them have the URL chrome://gfxsanity/content/sanitytest.html , which I think is some kind of self testing the graphics code does. Looking at 76 only, we have about 7600 shutdownkill hang reports, and around 1000 crash reports with the sanity test URL. In fact, it looks like almost all of the crash reports with sanitytest URLs (all but 6!) are hangs. Maybe there's nothing to be done, but maybe that's something worth investigating.
Here's a couple of examples: