Don't change Document load progress in any page dismissal events.

This can confuse the logic for blocking modal dialogs.

BUG=536652

Review URL: https://codereview.chromium.org/1373113002

Cr-Commit-Position: refs/heads/master@{#351419}
WeakPtrWillBeRawPtr<Document> Document::contextDocument()
{
    if (m_contextDocument)
        return m_contextDocument;
    if (m_frame) {
#if ENABLE(OILPAN)
        return this;
#else
        return m_weakFactory.createWeakPtr();
#endif
    }
    return nullptr;
}

WeakPtrWillBeRawPtr<Document> Document::contextDocument()
{
    if (m_contextDocument)
        return m_contextDocument;
    if (m_frame) {
#if ENABLE(OILPAN)
        return this;
#else
        return m_weakFactory.createWeakPtr();
#endif
    }
    return nullptr;
}
