Add multi_process_lock to base

Platform abstraction for a shared lock between processes. The process
that owns the lock will release it on exit even if exit is due
to a crash.

For cloud-print and remoting we want to be able to have a singleton service-process that can run independently of the browser process. This service process will communicate with the browser process via the standard IAC channels, but we want to have a way of signaling to other processes that a) there is a service-process running and b) that it is in a state where it is ready to be communicated with. The multi_process_lock class is intended to work as a simple flag that can be queried from multiple processes. If the service-process should crash, we would like the flag to be cleared automatically so that there is never confusion about the state of the service-process.

Other approaches considered for some Unix/Mac:
- Standard unix domain sockets depend on the file system and don't clean up properly in the case of a crash.
- Shared memory on unix depend on the file system and don't clean up properly in the case of a crash.
- System V semaphores on unix again depend on the file system.
- named_mach_ports on Mac OS. Bootstrap_register_name has been deprecated on 10.6, so we are doing essentially the same thing using CFMessagePort.

On Windows it is implemented as an event.
On Mac it is implemented using a CFMessagePort name.
On Linux it is implement using an abstract name port socket.

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/4721001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@66323 0039d316-1c4b-4281-b951-d872f2087c98

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