This patch centralizes all of the pref-checking code for e10s-multi in a
single function. It is intended to be used throughout the codebase to see if
e10s-multi is "on". It also introduces dom.ipc.multiOptOut, which can be set
by the user to indicate that they do not want to participate in the e10s-multi
experiment.
MozReview-Commit-ID: Kyq1fqNzwue
--HG--
extra : rebase_source : 321a44fb5909c067a20dbb3b739175ba08569a5f