Right now, e10s defaults to "on" in Nightly via the default pref
browser.tabs.remote.autostart.2=true. The reftest harness ignores that pref
and attempts to enable it by setting browser.tabs.remote.autostart to true (as
well as disabling the checks for non-mpc-compatible extensions). There's a
bug, however, and we overwrite the value for .autostart by reading
reftest-prefernces.js (which unconditionally sets .autostart to false).
Therefore, the existing code works because it ignores the .autostart.2 pref
and whether we enable e10s is entirely controlled by setting
extensions.e10sBlocksEnabling (the reftest extension is not mpc-compatible).
With this change, we unconditionally set .remote to the value that we want
(and we trust that the only non-mpc-compatible extensions are specially vetted
and part of our build system) so that things work as advertised on the box.
MozReview-Commit-ID: Li5N4NP4PwD
53693798ec