Change HTTP Basic Auth messaging to be resistent to long realm strings, and to clarify that the realm portion of the message is coming from a remote server.

      BUG=79426
TEST=none
Review URL: http://codereview.chromium.org/6905140

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@83731 0039d316-1c4b-4281-b951-d872f2087c98