See https://wiki.mozilla.org/Quantum/DOM#Labeling for the story.
No calls to NS_DispatchTo(Main|Current)Thread under layout/ipc/. Calls to nsIRunnable::Dispatch under layout/ipc/. http://searchfox.org/mozilla-central/search?q=symbol:_ZN14nsIEventTarget8DispatchEP11nsIRunnablej&redirect=false
Here to noted that bug 1333962 may bake care of vsync relatives in layout/ipc/
vsyncEvent runnable is dispatched from PBackground thread on main process to client process vsync thread. Currently there is no need to label this runnable.  http://searchfox.org/mozilla-central/source/layout/ipc/VsyncParent.cpp#56