In all of these cases the fixed buffer has the same lifetime as the string
object, so we can use nsAuto[C]String for simplicity.
For the 128-length ones in dom/xul/ I just switched to the default of 64 for
simplicity, because the choice of 128 didn't seem that important. (These code
paths weren't hit when I started the browser and opened a few sites.)
Finally, the patch also changes LoggingIdString to use
nsAutoCStringN<NSID_LENGTH>, similar to NullPrincipalURI.