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 RenderViewImpl::ApplyWebPreferencesInternal(const WebPreferences& prefs,
                                                 blink::WebView* web_view) {
  ApplyWebPreferences(prefs, web_view);
}

void RenderViewImpl::ApplyWebPreferencesInternal(const WebPreferences& prefs,
                                                 blink::WebView* web_view) {
  ApplyWebPreferences(prefs, web_view);
}
