Move from UnusedSlotXX to REMOVEDOldFeature in UseCounter.h

The previous system would end up with either out-of-order slots, which
makes it hard to find the next free slot, or would require frequent
re-ordering, which makes patches harder to write and review.

Some digging was done to find the original names for each slot.

BUG=none

Review URL: https://codereview.chromium.org/132983005

git-svn-id: svn://svn.chromium.org/blink/trunk@165277 bbb929c8-8fbe-4397-9dbb-9b2b20218538