Implicitly bypass localhost when proxying requests.

This aligns Chrome's behavior with the Windows and macOS proxy resolvers (but not Firefox).

Concretely:
 * localhost names (as determined by net::IsLocalhost) now implicitly bypass the proxy
 * link-local IP addresses implicitly bypass the proxy

The implicit rules are handled by ProxyBypassRules, and it is possible to override them when manually configuring proxy settings (but not when using PAC or auto-detect).

This change also adds support for the "<-loopback>" proxy bypass rule, with similar semantics as it has on Windows (removes the implicit bypass rules for localhost and link-local).

The compatibility risk of this change should be low as proxying through localhost was not universally supported. It is however an idiom used in testing (a number of our own tests had such a dependency). Impacted users can use the "<-loopback>" bypass rule as a workaround.

Bug: 413511, 899126, 901896
Change-Id: I263ca21ef9f12d4759a20cb4751dc3261bda6ac0
Reviewed-on: https://chromium-review.googlesource.com/c/1303626
Commit-Queue: Eric Roman <eroman@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606112}
  HttpProxyScriptBrowserTest() {
    http_server_.ServeFilesFromSourceDirectory("chrome/test/data");
  }

  HttpProxyScriptBrowserTest() {
    http_server_.ServeFilesFromSourceDirectory("chrome/test/data");
  }
