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.

This CL is a fixed version of r155823, which was reverted because it was causing
an assertion in clusterfuzz.

R=abarth, adamk
BUG=265009, 271131

Review URL: https://codereview.chromium.org/117313002

git-svn-id: svn://svn.chromium.org/blink/trunk@164029 bbb929c8-8fbe-4397-9dbb-9b2b20218538