Rework socks5 authentication.

[riastradh-brave]

• Work around Chromium's new in-line constructor size limits.

• Get base::StringPiece forward declaration in url_auth_util.h.

• Use `#if !defined(BRAVE_CHROMIUM_BUILD)', not #if 0.

• Override and include, rather than patch, net_log_event_type_list.h.

• Override and include host_port_pair.cc to reduce BUILD.gn diffs.

• Override and include url_util.cc rather than add url_auth_util.cc.

• Override and include socks5_client_socket.cc.

• Don't patch HostPortPair(host, port) constructor.

  Instead, use a different constructor in proxy_server.cc.

  This adds another patch to our maintenance burden, but it avoids
  changing the semantics of (and adding parsing overhead to) the
  constructor that is used in a bazillion places all over Chromium.

This is a replacement for #290, in which during squashing or something I lost two files 9socks5_client_socket.cc/h) that were critical to the whole thing.

fix brave/brave-browser#666

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.

Test Plan:

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions

[riastradh-brave]

Redone in #311.