Revert "Simplify DocumentOrderedMap"

This reverts commit 52bd7a87201e0b4ee917af0708d1d0a0d871ba8e.

This CL caused a performance regression in getElementById:

BUG=272700
TBR=adamk@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22968004

git-svn-id: svn://svn.chromium.org/blink/trunk@156115 bbb929c8-8fbe-4397-9dbb-9b2b20218538