[mojo-core] Validate data pipe endpoint metadata

Ensures that we don't blindly trust specified buffer size and offset
metadata when deserializing data pipe consumer and producer handles.

Bug: 877182
Change-Id: I30f3eceafb5cee06284c2714d08357ef911d6fd9
Reviewed-on: https://chromium-review.googlesource.com/1192922
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586704}
HandleSignalsState DataPipeProducerDispatcher::GetHandleSignalsState() const {
  base::AutoLock lock(lock_);
  return GetHandleSignalsStateNoLock();
}

HandleSignalsState DataPipeProducerDispatcher::GetHandleSignalsState() const {
  base::AutoLock lock(lock_);
  return GetHandleSignalsStateNoLock();
}
