Simplify DocumentOrderedMap

This CL simplifies DocumentOrderedMap's internal state. Rather than keeping a
map separately from the duplicate count, we now keep the duplicate count in the
same map.

R=adamk@chromium.org
BUG=265009

Review URL: https://chromiumcodereview.appspot.com/22634007

git-svn-id: svn://svn.chromium.org/blink/trunk@155823 bbb929c8-8fbe-4397-9dbb-9b2b20218538