IPC: Generalize FileDescriptorSet to MessageAttachmentSet

IPC: Generalize FileDescriptorSet to MessageAttachmentSet

This is a preparation for crbug.com/448190, where we're going
to allow Mojo MessagePipe being sent over Chrome IPC.

The plan is that we'll extend existing FileDescriptorSet to
carry both platform handles and Mojo's MessagePipes, then
let ChannelMojo take care of these handles.

As the first step, this change renames FileDescriptorSet to
MessageAttachmentSet and moves it out from the OS_POSIX guard.
Although this change doesn't do anything significant, we'll
generalize it to carry both platform files and mojo handles
eventually.

R=agl@chromium.org, viettrungluu@chromium.org
BUG=448190
Committed: https://crrev.com/4b5c28e2f3e3400b5e91129704368845688d40fe
Cr-Commit-Position: refs/heads/master@{#311546}

[agl, 2015-01-14 18:58:10]

lgtm

[Hajime Morrita, 2015-01-14 19:21:33]

The CQ bit was checked by morrita@chromium.org

[Hajime Morrita, 2015-01-14 19:21:40]

Thanks!

[commit-bot: I haz the power, 2015-01-14 19:22:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/835873004/20001

[commit-bot: I haz the power, 2015-01-14 21:17:47]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-14 21:18:46]

Patchset 2 (id:??) landed as
https://crrev.com/4b5c28e2f3e3400b5e91129704368845688d40fe
Cr-Commit-Position: refs/heads/master@{#311546}