"In particular, subframe navigations are back-forward" -> Did you mean, "In particular, subframe back-forward navigations are browser-initiated"?

Yes indeed. I meant: "In particular, back-forward subframe navigations are browser-initiated, meaning that they won't execute BeforeUnload before starting, whild they currently do."

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3f8ab9c3ab1b4471ba67b261e73753686777f21d

commit 3f8ab9c3ab1b4471ba67b261e73753686777f21d
Author: clamy <clamy@chromium.org>
Date: Thu Jan 26 14:42:14 2017

Execute BeforeUnload on subframe browser-initiated navigations

This CL ensures we will wait on the BeforeUnload event to execute in teh
renderer while doing a browser-initiated subframe navigations. This
ensures that BeforeUnload will execute on history navigations in
subframes when PlzNavigate is enabled (as is the case in the current
architecture).

BUG= 683891 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2655433002
Cr-Commit-Position: refs/heads/master@{#446325}

[modify] https://crrev.com/3f8ab9c3ab1b4471ba67b261e73753686777f21d/content/browser/frame_host/navigator_impl_unittest.cc
[modify] https://crrev.com/3f8ab9c3ab1b4471ba67b261e73753686777f21d/content/browser/frame_host/render_frame_host_impl.cc
[modify] https://crrev.com/3f8ab9c3ab1b4471ba67b261e73753686777f21d/content/renderer/render_frame_impl.cc
[modify] https://crrev.com/3f8ab9c3ab1b4471ba67b261e73753686777f21d/third_party/WebKit/LayoutTests/FlagExpectations/enable-browser-side-navigation