Remove RenderTextControlPlaceholder

It was introduced to accomodate an ASSERT, which on reflection is
overdoing it a bit. Move the special-casing to the ASSERT and one
day make text control placeholders fit better into the RenderTree
model so they can be treated like normal children.

BUG=

Review URL: https://codereview.chromium.org/149053004

git-svn-id: svn://svn.chromium.org/blink/trunk@166104 bbb929c8-8fbe-4397-9dbb-9b2b20218538