ipc: Move methods into AttachmentBroker interface.

Consumers of AttachmentBroker typically get an instance with
AttachmentBroker::GetGlobal(). In order to use subclass methods, they had to
downcast. Move those subclass methods into the AttachmentBroker interface
removes the need to downcast.

BUG=535711

Review URL: https://codereview.chromium.org/1420763002

Cr-Commit-Position: refs/heads/master@{#355364}
6 files changed