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This is one of a series of three CLs adding more uniformity
to referrer controls' implementation. The switch's functionality
remains accessible via disabling the enable_referrers pref.

The changes in this CL comprise:
1. Removing the switch
2. Removing the previous calls directly checking the switch, rather than
the enable_referrers pref. Happily, these checks of the switch were both
redundant with the later check of enable_referrers performed in
NetworkDelegate::NotifyBeforeURLRequest.
3. A nominal amount of refactoring of some dead/nearly-dead referrer-related
code:
  - network::ComputeReferrer removed from loader_util.cc (after removing
the --no-referrers switch, this method now duplicates GURL::GetAsReferrer).
  - content::Referrer::SetReferrerForRequest removed (it had no callers).

R=gab, falken

Bug: 1004486
Change-Id: I00bc503d0954700565047a188bbb9842c4d4bc9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808144
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699403}