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 ApplyBlinkSettings(const base::CommandLine& command_line,
                        WebSettings* settings) {
  if (!command_line.HasSwitch(switches::kBlinkSettings))
    return;

  std::vector<std::string> blink_settings = base::SplitString(
      command_line.GetSwitchValueASCII(switches::kBlinkSettings),
      ",", base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
  for (const std::string& setting : blink_settings) {
    size_t pos = setting.find('=');
    settings->SetFromStrings(
        blink::WebString::FromLatin1(setting.substr(0, pos)),
        blink::WebString::FromLatin1(
            pos == std::string::npos ? "" : setting.substr(pos + 1)));
  }
}

void ApplyBlinkSettings(const base::CommandLine& command_line,
                        WebSettings* settings) {
  if (!command_line.HasSwitch(switches::kBlinkSettings))
    return;

  std::vector<std::string> blink_settings = base::SplitString(
      command_line.GetSwitchValueASCII(switches::kBlinkSettings),
      ",", base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
  for (const std::string& setting : blink_settings) {
    size_t pos = setting.find('=');
    settings->SetFromStrings(
        blink::WebString::FromLatin1(setting.substr(0, pos)),
        blink::WebString::FromLatin1(
            pos == std::string::npos ? "" : setting.substr(pos + 1)));
  }
}
