Revert Blink r154194.

Revert "Manage FloatingObjects using hashmaps" due to performance regressions.

TBR=inferno@chromium.org
BUG=260286

Review URL: https://chromiumcodereview.appspot.com/22440002

git-svn-id: svn://svn.chromium.org/blink/trunk@155630 bbb929c8-8fbe-4397-9dbb-9b2b20218538