This should also track Chrome-Proxy headers needed for the fetch.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/af52374798f2a921b2fb943727065b339f036e0a

commit af52374798f2a921b2fb943727065b339f036e0a
Author: Ryan Sturm <ryansturm@chromium.org>
Date: Wed Jan 16 00:15:45 2019

Initial CL for moving HTTPS server previews to URLLoader

This CL introduces a feature and basic UMA for a
URLLoaderRequestInterceptor for HTTPS lite page previews.

Bug: 921740
Change-Id: I1173e79e7fb53d2d9a938bf891a023048e745126
Reviewed-on: https://chromium-review.googlesource.com/c/1410163
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622909}
[modify] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/chrome/browser/BUILD.gn
[modify] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/chrome/browser/chrome_content_browser_client.cc
[modify] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/chrome/browser/previews/previews_lite_page_decider.cc
[modify] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/chrome/browser/previews/previews_lite_page_navigation_throttle.cc
[add] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/chrome/browser/previews/previews_lite_page_url_loader_interceptor.cc
[add] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/chrome/browser/previews/previews_lite_page_url_loader_interceptor.h
[add] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/chrome/browser/previews/previews_lite_page_url_loader_interceptor_unittest.cc
[modify] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/chrome/test/BUILD.gn
[modify] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/components/previews/core/previews_features.cc
[modify] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/components/previews/core/previews_features.h
[modify] https://crrev.com/af52374798f2a921b2fb943727065b339f036e0a/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e6bfde824c09f7b6cd421b5e239d59745e4d0da5

commit e6bfde824c09f7b6cd421b5e239d59745e4d0da5
Author: Ryan Sturm <ryansturm@chromium.org>
Date: Tue Jan 22 22:11:26 2019

Laying down more Previews URLLoader skeleton code

A lot of this is non functional, and the code still just falls back to
default, but it lays down the general approach and leaves room to add
various aspects one piece at a time.

Bug: 921740
Change-Id: I8ca21f1ca2a9ddee77f1f346ca4612a0a4719df3
Reviewed-on: https://chromium-review.googlesource.com/c/1416772
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624959}
[modify] https://crrev.com/e6bfde824c09f7b6cd421b5e239d59745e4d0da5/chrome/browser/BUILD.gn
[add] https://crrev.com/e6bfde824c09f7b6cd421b5e239d59745e4d0da5/chrome/browser/previews/previews_lite_page_redirect_url_loader.cc
[add] https://crrev.com/e6bfde824c09f7b6cd421b5e239d59745e4d0da5/chrome/browser/previews/previews_lite_page_redirect_url_loader.h
[add] https://crrev.com/e6bfde824c09f7b6cd421b5e239d59745e4d0da5/chrome/browser/previews/previews_lite_page_serving_url_loader.cc
[add] https://crrev.com/e6bfde824c09f7b6cd421b5e239d59745e4d0da5/chrome/browser/previews/previews_lite_page_serving_url_loader.h
[modify] https://crrev.com/e6bfde824c09f7b6cd421b5e239d59745e4d0da5/chrome/browser/previews/previews_lite_page_url_loader_interceptor.cc
[modify] https://crrev.com/e6bfde824c09f7b6cd421b5e239d59745e4d0da5/chrome/browser/previews/previews_lite_page_url_loader_interceptor.h
[modify] https://crrev.com/e6bfde824c09f7b6cd421b5e239d59745e4d0da5/chrome/browser/previews/previews_lite_page_url_loader_interceptor_unittest.cc