Add GetAppOutput function, a better replacement for popen.

It will replace popen call in chrome_process_util_linux.

I don't see much benefit in having a Windows implementation
(not many useful programs you can launch and get output),
so POSIX-only.

Review URL: http://codereview.chromium.org/67226

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@13920 0039d316-1c4b-4281-b951-d872f2087c98

• base/process_util.h[diff]
• base/process_util_posix.cc[diff]
• base/process_util_unittest.cc[diff]