This combines Map::DoneInobjectSlackTracking and Map::ConstructionCount into one more generic 4-bit counter.

Counter values from 15 down to 8 are used for in-object slack tracking, values from 7 down to 0 are free to be used for a new counter when in-object slack tracking is inactive.

Review URL: https://codereview.chromium.org/767253002

Cr-Commit-Position: refs/heads/master@{#25689}