Prevent renderer initiated back navigation to cancel a browser one.

Renderer initiated back/forward navigations must not be able to cancel ongoing
browser initiated navigation if they are not user initiated.

Note: 'normal' renderer initiated navigation uses the
FrameHost::BeginNavigation() path. A code similar to this patch is done
in NavigatorImpl::OnBeginNavigation().

Test:
-----

Added: NavigationBrowserTest.
 * HistoryBackInBeforeUnload
 * HistoryBackInBeforeUnloadAfterSetTimeout
 * HistoryBackCancelPendingNavigationNoUserGesture
 * HistoryBackCancelPendingNavigationUserGesture

Fixed:
 * (WPT) .../the-history-interface/traverse_the_history_2.html
 * (WPT) .../the-history-interface/traverse_the_history_3.html
 * (WPT) .../the-history-interface/traverse_the_history_4.html
 * (WPT) .../the-history-interface/traverse_the_history_5.html

Bug: 879965
Change-Id: I1a9bfaaea1ffc219e6c32f6e676b660e746c578c
Reviewed-on: https://chromium-review.googlesource.com/1209744
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592823}
void WebContentsImpl::FullscreenFrameSetUpdated() {
  if (fullscreen_frames_.empty()) {
    current_fullscreen_frame_ = nullptr;
    return;
  }

  RenderFrameHostImpl* new_fullscreen_frame = *std::max_element(
      fullscreen_frames_.begin(), fullscreen_frames_.end(), FrameCompareDepth);

  if (new_fullscreen_frame == current_fullscreen_frame_)
    return;
  current_fullscreen_frame_ = new_fullscreen_frame;

  for (auto& observer : observers_)
    observer.DidAcquireFullscreen(new_fullscreen_frame);

  if (display_cutout_host_impl_)
    display_cutout_host_impl_->DidAcquireFullscreen(new_fullscreen_frame);
}

void WebContentsImpl::FullscreenFrameSetUpdated() {
  if (fullscreen_frames_.empty()) {
    current_fullscreen_frame_ = nullptr;
    return;
  }

  RenderFrameHostImpl* new_fullscreen_frame = *std::max_element(
      fullscreen_frames_.begin(), fullscreen_frames_.end(), FrameCompareDepth);

  if (new_fullscreen_frame == current_fullscreen_frame_)
    return;
  current_fullscreen_frame_ = new_fullscreen_frame;

  for (auto& observer : observers_)
    observer.DidAcquireFullscreen(new_fullscreen_frame);

  if (display_cutout_host_impl_)
    display_cutout_host_impl_->DidAcquireFullscreen(new_fullscreen_frame);
}
