Bug 1384835 (part 3) - Remove the Preferences::Get*CString() variants that return nsAdoptingCString. r=froydnj.

This is similar like the previous patch, but for the 8-bit string variants.
Also, it changes assignment to Adopt() in GetCString() and GetDefaultCString()
to avoid an extra copy.

--HG--
extra : rebase_source : eba805c3a7b809d5ccd6e853b1c9010db9477667