Remove IPC::BrokerableAttachment.

With only ChannelMojo in use, the distinction between brokerable and
non-brokerable attachments no longer makes sense. This CL removes that
distinction by removing BrokerableAttachment and flattening the
hierarchy of attachment types.

This also trims some POSIX-specific parts of IPC::MessageAttachmentSet.

BUG=659448

Review-Url: https://codereview.chromium.org/2494943002
Cr-Commit-Position: refs/heads/master@{#432153}
32 files changed