Disable frame navigations during DocumentLoader detach in FrameLoader::startLoad

BUG=613266

Review-Url: https://codereview.chromium.org/2006033002
Cr-Commit-Position: refs/heads/master@{#396241}
bool isEnterKeyKeydownEvent(Event* event)
{
    return event->type() == EventTypeNames::keydown && event->isKeyboardEvent() && toKeyboardEvent(event)->keyIdentifier() == "Enter" && !toKeyboardEvent(event)->repeat();
}

bool isEnterKeyKeydownEvent(Event* event)
{
    return event->type() == EventTypeNames::keydown && event->isKeyboardEvent() && toKeyboardEvent(event)->keyIdentifier() == "Enter" && !toKeyboardEvent(event)->repeat();
}
