Append trailer data to serialized messages.

[chromium-wpt-export-bot]

This reserves space in the Blink envelope for an offset to trailer data appended to the message which stores summary information about the message without needing to deserialize it.

This allows quickly assessing which WebIDL interfaces must be exposed for the message to be deserialized successfully, which means a messageerror event can be dispatched instead of a message event, even without eagerly deserializing (as the spec suggests).

This is a major change that requires a bump to the serialization format and would require care to revert in the future.

Bug: 1341844,1366065 Change-Id: I45904ff3f6270ac0e31c49dfae45c72c7e0d135b Reviewed-on: https://chromium-review.googlesource.com/3758405 WPT-Export-Revision: 11820bff2d36fbf654f2821aeda3f50518a864c3