Stub in more IPC attachment brokering functionality.

This CL fills in most of the logic for sending a Windows HANDLE from a
non-broker process to the broker process. This consists of several small
changes:

- Create the new IPC messages AttachmentBrokerMsg_WinHandleHasBeenBrokered and
AttachmentBrokerMsg_RequestBrokerageOfWinHandle.
- Add a sender_ member variable to AttachmentBrokerWin.
- Define the wire format for HandleAttachmentWin.
- Add logic to ChannelWin to send
AttachmentBrokerMsg_RequestBrokerageOfWinHandle for each HANDLE attachment.

BUG=466437

Review URL: https://codereview.chromium.org/1188923003

Cr-Commit-Position: refs/heads/master@{#337689}

1 files changed, 1 insertions, 0 deletions

diff --git a/ipc/ipc_message_start.h b/ipc/ipc_message_start.h
index b37c5c9..f5538a9 100644
--- a/ipc/ipc_message_start.h
+++ b/ipc/ipc_message_start.h
@@ -123,6 +123,7 @@ enum IPCMessageStart {
   ContentSettingsMsgStart,
   ChromeAppBannerMsgStart,
   SafeJsonParserMsgStart,
+  AttachmentBrokerMsgStart,
   LastIPCMsgStart  // Must come last.
 };