nsBase64Encoder::Finish currently works by calling PL_Base64Encode to
allocate a base64-encoded string, then Assign()'ing that to the result
string, another allocation. mozilla::Base64Encode enables us to base64
encode directly into the result string with a single allocation, saving
an allocation. (Base64Encode is also slightly more efficient, because
we don't have to do a strlen() on the string being Assign()'d.) Let's
use Base64Encode instead.
It's silly to use prmem.h within Firefox code given that in our configuration
its functions are just wrappers for malloc() et al. (Indeed, in some places we
mix PR_Malloc() with free(), or malloc() with PR_Free().)
This patch removes all uses, except for the places where we need to use
PR_Free() to free something allocated by another NSPR function; in those cases
I've added a comment explaining which function did the allocation.
