Always use FrameNavigationDisabler during DocumentLoader detach.

BUG=617495

Review-Url: https://codereview.chromium.org/2079473002
Cr-Commit-Position: refs/heads/master@{#400558}
DEFINE_TRACE(FrameLoader)
{
    visitor->trace(m_frame);
    visitor->trace(m_progressTracker);
    visitor->trace(m_documentLoader);
    visitor->trace(m_provisionalDocumentLoader);
    visitor->trace(m_currentItem);
    visitor->trace(m_provisionalItem);
    visitor->trace(m_deferredHistoryLoad);
}

DEFINE_TRACE(FrameLoader)
{
    visitor->trace(m_frame);
    visitor->trace(m_progressTracker);
    visitor->trace(m_documentLoader);
    visitor->trace(m_provisionalDocumentLoader);
    visitor->trace(m_currentItem);
    visitor->trace(m_provisionalItem);
    visitor->trace(m_deferredHistoryLoad);
}
