Actually, for adding NoScript Preview (or other client side previews), we will need to make the PreviewsState update decision in I/O thread upon starting the response and plumb single outcome to both renderer (for WebURLResponse) and UI thread (NavigationData for PreviewsInfoBar). It may be that ChromeResourceDispatcherHostDelegate::GetNavigationData() is the better place to make this update with the current layering constraints. [Alternatively, we could consider moving DataReductionProxyNetworkDelegate (and the previews component) out of layered core components and into content aware code.]

dougarnett, is this correctly marked as a P1?

Setting as P2 - only an issue for launching NoScript

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1921bb351242e3ada575cc85858957f9c56106a8

commit 1921bb351242e3ada575cc85858957f9c56106a8
Author: Doug Arnett <dougarnett@chromium.org>
Date: Wed Dec 13 06:59:34 2017

Makes navigation commit-time decision for previews in one location

This is a refactor to consolidate the commit-time previews decision
in one place (in ChromeResourceDispatcherHostDelegate) for both
server-determined and client-determined previews with propagation
of decision to renderer (via PreviewsState in mainframe response
info) and UI thread (via PreviewsUserData).

Bug:  782922 , 725645
Change-Id: I8f7e5ef3a8a45b9ba2e1bbc07f09bfb9670e16b1
Reviewed-on: https://chromium-review.googlesource.com/807305
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Scott Little <sclittle@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523705}
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/chrome/browser/loader/chrome_resource_dispatcher_host_delegate.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/chrome/browser/loader/chrome_resource_dispatcher_host_delegate.h
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/chrome/browser/loader/chrome_resource_dispatcher_host_delegate_unittest.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/chrome/browser/previews/previews_infobar_tab_helper.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/chrome/browser/previews/previews_infobar_tab_helper_unittest.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/data_reduction_proxy/content/browser/content_lofi_decider.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/data_reduction_proxy/content/browser/content_lofi_decider.h
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/data_reduction_proxy/content/browser/content_lofi_decider_unittest.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/data_reduction_proxy/content/renderer/content_previews_render_frame_observer.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/data_reduction_proxy/content/renderer/content_previews_render_frame_observer.h
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/data_reduction_proxy/content/renderer/content_previews_render_frame_observer_unittest.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/previews/content/previews_content_util.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/previews/content/previews_content_util.h
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/previews/content/previews_content_util_unittest.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/previews/core/previews_user_data.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/components/previews/core/previews_user_data.h
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/content/browser/loader/mojo_async_resource_handler_unittest.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/content/browser/loader/resource_dispatcher_host_browsertest.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/content/browser/loader/resource_dispatcher_host_impl.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/content/public/browser/resource_dispatcher_host_delegate.cc
[modify] https://crrev.com/1921bb351242e3ada575cc85858957f9c56106a8/content/public/browser/resource_dispatcher_host_delegate.h