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

Committed: https://crrev.com/f6e03ce56c4d2370b79d0c3dd4ceb89cf5528e56
Review-Url: https://codereview.chromium.org/2494943002
Cr-Original-Commit-Position: refs/heads/master@{#432153}
Cr-Commit-Position: refs/heads/master@{#434099}