Stringify minidump stream_type constants in minidump_dump output

I frequently find myself looking at minidump_dump output and deciphering
constants. This helps for the stream directory, at least. You can see the new
output in the testdata diff. I'm not wild about having a hardcoded list in a
function like this, but there's no sane way to do this in C++ and I didn't want
to munge the header into a bunch of macrology.

[Ted Mielczarek]

I forgot to send mail, as usual.

[Mark Mentovai]

I would have been OK with a stringify macro too, by the way.

LGTM

https://breakpad.appspot.com/3704002/diff/1/src/processor/minidump.cc
File src/processor/minidump.cc (right):

https://breakpad.appspot.com/3704002/diff/1/src/processor/minidump.cc#newcode...
src/processor/minidump.cc:4775: printf("  stream_type        = %d (%s)\n",  
directory_entry->stream_type,
I think we should use 0x%x here like we do when we dump the stream map.