nsCString::GetBuffer() -> nsCString::get()

[Peter Annema]

As part of the upcoming string lib beautifications, I have a bug[0] on
removing all nsCString::GetBuffer() calls and replacing them with
nsCString::get().

I hope to land these changes soon, and I'm sure I'll be breaking some
patched trees out there. They shouldn't be hard to fix however, just
replace |GetBuffer()| with |get()|.

Sincerely,

  jag

[0] http://bugzilla.mozilla.org/show_bug.cgi?id=64016