Move PreconnectManager into content.

This change moves the following files to //content/public/browser
- preconnect_manager.{cc,h}

This change moves the following files to //content/browser/preloading
- preconnect_manager_impl.{cc,h}
- preconnect_manager_impl_unittest.cc
- resolve_host_client_impl.{cc,h}

It deletes proxy_lookup_client_impl.{cc,h} in //chrome and replaces its
use with the one in //content. To do this, it adds an extra constructor
overload.

There should be no functional change in this CL.

The purpose of this change is to allow the use of PreconnectManager by
//android_webview/ so that we can expose the ability to preconnect to
certain URLs in the WebView API.

Bug: 432235374
Change-Id: I63e8ea83c20257ea6978ef8192b21566aa309cd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6788473
Reviewed-by: Keita Suzuki <suzukikeita@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1494309}