Revert "Avoid N^2 walk placing renderers when building the render tree"

Reverting r154053. This resulted in a big perf hit :(

BUG=245478,259926
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/18464005

git-svn-id: svn://svn.chromium.org/blink/trunk@154161 bbb929c8-8fbe-4397-9dbb-9b2b20218538