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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

Review URL: https://codereview.chromium.org/835873004

Cr-Commit-Position: refs/heads/master@{#311546}