GitLab

* Remove all unnecessary IsSpdyAlternative() checks.
  IsSpdyAlternative() implies origin_url_.SchemeIs(url::kHttpsScheme),
  see DCHECK in constructor.
* Inline now const |spdy_certificate_error_|.
* Inline and remove SwitchToSpdyMode().  This method was only used twice out of
  four potential places.  Removal makes sense as the method is trivial, and
  there are no corresponding methods for |using_spdy_ = false|, |using_quic_ =
  true|, and |using_quic_ = false| assignments.
* Use url_constants instead of string literals for schemes (http, https, ftp,
  and wss).  There has already been two uses of such constants.
* Remove unused |destination| local variable.
* Fix some comments.
* git cl lint: Remove unnecessary semicolon, add includes.

BUG=475060

Review-Url: https://codereview.chromium.org/2359153003
Cr-Commit-Position: refs/heads/master@{#420725}