Bug 946407: Reimplement DumpMemoryInfoToTempDir in terms of GetReports. r=njn

This winds up exposing things in the nsIMemoryReporterManager interface
that arguably don't belong at that level of abstraction -- "minimize
memory usage first" and DMD -- in order to take advantage of the
infrastructure that GetReports already has for managing the child
processes.
Bug 946407: Disable sandbox when DMDing. r=njn r=kang

See also bug 956961.