Fix cr-bash-helpers for Mac w/ Homebrew, which doesn't have realpath.

readlink is in gnu coreutils, which should be more widely available. On my
machine, the gnu version is prefixed with 'g', while 'readlink' is a crippled
Mac version without the -e option.

Review URL: https://codereview.chromium.org/144243004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248853 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248853