NaCl: Add assertion that NaClMainForChromium() does not return
NaCl: Add assertion that NaClMainForChromium() does not return

NaClMainForChromium() should do an _exit() when NaCl untrusted code
does an exit syscall.  I will change NaClMainForChromium() on the NaCl
side to do this.

Update the function prototype to match the NaCl-side change.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2083
TEST=run_ppapi_crash_browser_test currently triggers this, before the
  NaCl-side change, because its CHECK() call happens to do an _exit()

Review URL: http://codereview.chromium.org/7500004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@93962 0039d316-1c4b-4281-b951-d872f2087c98

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