ipc: Make a new class PlaceholderBrokerableAttachment.
Instances of this class need to be replaced before the message can be
dispatched. This mechanism is a lot cleaner than the previous mechanism, which
had an "unbrokered" state for every type of BrokerableAttachment, which had to
be mutated before being dispatched.
BUG=493414
Review URL: https://codereview.chromium.org/1317093007
Cr-Commit-Position: refs/heads/master@{#348952}
CrOS-Libchrome-Original-Commit: 87351da8166440ba0978a3c74701aa8242b72295
10 files changed