AFAICT, we have to stop talking about docshells for bug 1152988 to make any kind of forward progress. This whole docshell business also makes tracing events in code that shouldn't know anything about docshells very funky, like in bug 1141614.

We could also just hide the docshell usage to non-inline ctor/dtor, so docshell part could be some main-thread only case. And this really depends on what devtools want to do with markers. Should the markers work across page navigations? What about calling document.write() after page load (that creates a new inner window object). But if per-global is really what is wanted here, great.

Victor's doing this in bug 1183219 for OTMT markers