POSIX: Rewrite IPC's interaction with FileDescriptor

POSIX: Rewrite IPC's interaction with FileDescriptor

The FileDescriptor API is clearly too hard to use. It's the only IPC
data type which is non-POD and serialising an invalid file descriptor
is fatal to Chrome on POSIX. The use of Maybe is possibly non-obvious
to non-functional programmers.

This patch merges Maybe and FileDescriptor so that serialising invalid
file descriptors is permitted and results in -1 at the other end.
(Serialising /closed/ a file descriptor is still fatal.) Also, it adds
a pointer in base/file_descriptor.h to instructions for its use with
IPC. Although it's generally bad practice to mention IPC in base, in
this case I cannot find another suitable location.

[agl, 2009-03-05 19:26:28]

darin: review
hclam: cc only

[darin (slow to review), 2009-03-05 19:42:12]

seems like nice cleanup to me.  LGTM

http://codereview.chromium.org/39208/diff/1/8
File chrome/common/ipc_message_utils.h (right):

http://codereview.chromium.org/39208/diff/1/8#newcode730
Line 730: } else {
nit: no need for else after return

[Alpha Left Google, 2009-03-05 20:39:38]

http://codereview.chromium.org/39208/diff/1/2
File base/file_descriptor_posix.h (right):

http://codereview.chromium.org/39208/diff/1/2#newcode27
Line 27: int fd;
Should this be type base::PlatformFile?