Added ability to run a command, getting output and exit status on POSIX.

Register protocol handler needs to check the return value of scripts it runs and
collects output from. There needs a variant of GetAppOutput which returns the
exit code of the application run, not just a success or failure, so this function
has been added.

BUG=83557
TEST=Unit test added


Review URL: http://codereview.chromium.org/7198034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@91527 0039d316-1c4b-4281-b951-d872f2087c98

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