Cleanup anonymous text objects that pass the document as a node.

This CL requires a pseudo-element to own the anonymous layout object in most of
cases. This doesn't switch yet to having the pseudo in LayoutObject::m_node.

This is preliminar work for display: contents pseudos.

BUG=657748

Review-Url: https://codereview.chromium.org/2754653002
Cr-Commit-Position: refs/heads/master@{#457121}
15 files changed