Does this block Issue 745942?

No. If AMP uses the GetPreviewsState method, it should respect DISABLE_LOFI_MODE already.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/006f2532ea48d3f3185baf7c800633aabfccfb96

commit 006f2532ea48d3f3185baf7c800633aabfccfb96
Author: Thanh Le <thanhdle@chromium.org>
Date: Wed Sep 13 17:38:04 2017

Change the enum name of DISABLE_LOFI_MODE to DISABLE_PREVIEWS since it disables all previews outside of general reload topic.

Bug:  707272 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: Ied478af0c8322a7d48494d5614695b4ef7c368d2
Reviewed-on: https://chromium-review.googlesource.com/663822
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Thanh Le <thanhdle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501680}
[modify] https://crrev.com/006f2532ea48d3f3185baf7c800633aabfccfb96/chrome/browser/previews/previews_infobar_delegate.cc
[modify] https://crrev.com/006f2532ea48d3f3185baf7c800633aabfccfb96/chrome/browser/previews/previews_infobar_delegate_unittest.cc
[modify] https://crrev.com/006f2532ea48d3f3185baf7c800633aabfccfb96/content/browser/frame_host/navigator_impl.cc
[modify] https://crrev.com/006f2532ea48d3f3185baf7c800633aabfccfb96/content/browser/loader/resource_dispatcher_host_browsertest.cc
[modify] https://crrev.com/006f2532ea48d3f3185baf7c800633aabfccfb96/content/public/browser/reload_type.h