Fix failing tests with ChannelMojo enabled.

This ChannelMojo to:
- take a ScopedMessagePipeHandle instead of a string token so an
  in-process renderer can be passed the message pipe directly;
- send brokered attachments as mojo handles; and
- offer messages to AttachmentBroker.

This also fixes and re-enables ipc_channel_mojo_unittest.cc.

BUG=579813

Committed: https://crrev.com/013cfed7ecf91b0700bec7147631d4fbedb6b64e
Cr-Commit-Position: refs/heads/master@{#380294}

Review URL: https://codereview.chromium.org/1768903002

Cr-Commit-Position: refs/heads/master@{#380360}
diff --git a/ipc/handle_attachment_win.cc b/ipc/handle_attachment_win.cc
index 39c4ef7..cb6a7bacf 100644
--- a/ipc/handle_attachment_win.cc
+++ b/ipc/handle_attachment_win.cc
@@ -24,6 +24,10 @@
   }
 }
 
+HandleAttachmentWin::HandleAttachmentWin(const HANDLE& handle,
+                                         FromWire from_wire)
+    : handle_(handle), permissions_(HandleWin::INVALID), owns_handle_(true) {}
+
 HandleAttachmentWin::HandleAttachmentWin(const WireFormat& wire_format)
     : BrokerableAttachment(wire_format.attachment_id),
       handle_(LongToHandle(wire_format.handle)),