Put app shim IPC socket in a temporary directory. (Mac)

The path to the App Shim Socket must be less than 104 characters. Since the
socket is inside the user data dir, the path is frequently too long in tests. It is
also very likely to be too long for any users that have a long username.

In this CL, the socket is always placed at:
"<DIR_TEMP>/XXXXXX/App Shim Socket"
where XXXXXX is replaced with a random suffix by mkdtemp.
A symlink to the socket is placed in "<udd>/App Shim Socket" (the old socket
path). The shim reads this symlink to find the actual socket.

BUG=240554

Review URL: https://codereview.chromium.org/66043003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250051 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250051