Convert DOMWrapperMap to use PersistentValueMap

BUG=
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/180363004

git-svn-id: svn://svn.chromium.org/blink/trunk@170974 bbb929c8-8fbe-4397-9dbb-9b2b20218538