Always use FrameNavigationDisabler during DocumentLoader detach.

BUG=617495

Review-Url: https://codereview.chromium.org/2079473002
Cr-Commit-Position: refs/heads/master@{#400558}
void FrameLoader::scheduleCheckCompleted()
{
    if (!m_checkTimer.isActive())
        m_checkTimer.startOneShot(0, BLINK_FROM_HERE);
}

void FrameLoader::scheduleCheckCompleted()
{
    if (!m_checkTimer.isActive())
        m_checkTimer.startOneShot(0, BLINK_FROM_HERE);
}
