Remove dead fields, found by scythe.

These were added for the memory tracking infrastructure we once had, and wasn't
deleted when that infrastructure went away.

BUG=none

Review URL: https://codereview.chromium.org/139783002

git-svn-id: svn://svn.chromium.org/blink/trunk@165158 bbb929c8-8fbe-4397-9dbb-9b2b20218538