Remove Unused measure DocumentTypeEntities

Intent to Remove:
https://groups.google.com/a/chromium.org/forum/?pli=1#!topic/blink-dev/ze27T2M682w

BUG=342302

Review URL: https://codereview.chromium.org/159513002

git-svn-id: svn://svn.chromium.org/blink/trunk@166898 bbb929c8-8fbe-4397-9dbb-9b2b20218538