This enables the use of ubi::Node in situations where a JSContext* is not
available, and paves the way for debugging utilities to dump shortest paths from
GC roots to a given ubi::Node that can be used while paused in a debugger.
Move major DevTools files to new directories using the following steps:
hg mv browser/devtools devtools/client
hg mv toolkit/devtools/server devtools/server
hg mv toolkit/devtools devtools/shared
No other changes are made.
