Fixes in base for [P]NaCl port of remoting client

Various minor fixes in base needed for [P]NaCl port of remoting client:
  - added base_i18n_untrusted target
  - md5 is included in base_untrusted.
  - base/metrics is compiled in base_untrusted (src/net depends on it).
  - enabled PNaCl build of base_untrusted
  - __builtin_extract_return_addr() is not allowed in PNaCl (validator
    rejects it).
  - sys_byteorder.h now works in NaCl.

BUG=276739

Review URL: https://codereview.chromium.org/116803005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242270 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242270