Add message serialize and deserialize associated endpoint handles.
Add new MessageV2Builder, add encode and decode for associated endpoint handles.
Modify tmpl to generate AssociatedInterfacePtr and use MessageV2Builder.
Add associated_interface_ptr.html layout test to test encoding/decoding.

BUG=695635

Review-Url: https://codereview.chromium.org/2820783002
Cr-Commit-Position: refs/heads/master@{#466531}