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to not take a request_url, since this is confusing for Alternate-Protocol
and Alt-Svc requests. Instead, take an enum which describes the type of
socket to be requested (FTP, SSL, or normal) and endpoint to which the
socket is to be connected.

Review URL: https://codereview.chromium.org/1051023003

Cr-Commit-Position: refs/heads/master@{#324196}