Always attach overflow controls to the right parent

Overflow controls like scrollbars and scroll corners can either be
parented by root transform layer (which optionally might exist) or the
overflow controls host layer (if the root transform layer does not).

Previously, overflow controls were parented to the first control parent
that they were attached to, and if the root transform layer got created,
they wouldn't be reparented to it.  This leads to raciness between
adding scrollbars and adding the root transform layer.

To fix this, always reparent these controls if the controlsParent has
changed.

BUG=361729

Review URL: https://codereview.chromium.org/266003004

git-svn-id: svn://svn.chromium.org/blink/trunk@173263 bbb929c8-8fbe-4397-9dbb-9b2b20218538