Correctly keep track of isolates for microtask execution

BUG=487155
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/1161823002

git-svn-id: svn://svn.chromium.org/blink/trunk@195985 bbb929c8-8fbe-4397-9dbb-9b2b20218538
void Document::clearWeakMembers(Visitor* visitor)
{
    if (m_axObjectCache)
        m_axObjectCache->clearWeakMembers(visitor);
}

void Document::clearWeakMembers(Visitor* visitor)
{
    if (m_axObjectCache)
        m_axObjectCache->clearWeakMembers(visitor);
}
