GitLab

This refactors the net->mojo adapter currently in the URL loader and moves it to the net_adapters file so it can be shared with the tcp_socket. An adapter going in the opposite direction (currently unused) is also added.

Makes some changes to the existing adapter: naming for clarify (since there is now one going in the opposite direction, things were getting pretty confusing), and some of the setup logic was moved into a static function so it can be shared.

Clarify requirements for calling EndWriteData (the old code called this in error cases as well).

Review URL: https://codereview.chromium.org/649783002

Cr-Commit-Position: refs/heads/master@{#299211}