More solid detection of browser process in chrome_process_util_linux.cc:

- use GetAppOutput instead of popen
- make unexpected conditions fatal (otherwise the tests using this code
  would mistakenly assume that there is no running browser process)

Also add a constant for SingletonSocket.

Make necessary adjustments to GetAppOutput - ignore stderr (because fuser
prints the file name to stderr and having stderr in |output| would
require more parsing).

Review URL: http://codereview.chromium.org/77031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@14091 0039d316-1c4b-4281-b951-d872f2087c98

• base/process_util.h[diff]
• base/process_util_posix.cc[diff]
• chrome/browser/process_singleton_linux.cc[diff]
• chrome/chrome.gyp[diff]
• chrome/common/chrome_constants.cc[diff]
• chrome/common/chrome_constants.h[diff]
• chrome/test/chrome_process_util_linux.cc[diff]
• chrome/test/chrome_process_util_uitest.cc[Added - diff]