Rename LayoutStateDisabler to ForceHorriblySlowRectMapping

LayoutState keeps track of a handful of fields, but the most
important part is providing a fast-mapping between renderers
and their repaint container. For parts where the logic doesn't
work, we disable LayoutState, which causes us to fall back on
walking up the tree and slowly calculating our rects.

Renaming LayoutStateDisabler, which sounds relatively innocuous
and which most people have no idea of the implications of to
ForceHorriblySlowRectMapping, with the hopes of striking fear
into any dev or reviewer who hopes to introduce one.

Review URL: https://codereview.chromium.org/333743002

git-svn-id: svn://svn.chromium.org/blink/trunk@176052 bbb929c8-8fbe-4397-9dbb-9b2b20218538