Electrolysis: multi-process behavior preffed off by default

bent checked in a change last week which disabled the IPC mechanism by
default in Electrolysis. We did this in order to get the unit-test machines
showing green on Tinderbox, to make sure we weren't introducing hidden
regressions. It also allows hackers to work on only tabs or plugins without
worrying about instability in the other code.

This should leave you with a usable browser by default with nightly builds
of Electrolysis. To enable separate plugin processes, set the
dom.ipc.plugins.enabled pref to true. To enable separate tab processes, set
the dom.ipc.tabs.enabled pref to true.

Note that there are still some unit-test failures/crashes that we need to
fix in the plugin code, so you might not have a very stable browser yet.

--BDS