GN: Enable embedder to decide whether or not the external V8 snapshot is enabled.

BUG=421063

Review URL: https://codereview.chromium.org/749213004

Cr-Commit-Position: refs/heads/master@{#25691}
MIPS: Refactor Map::ConstructionCount.

Port 35ff259b6344eed116936372f6861181c6afe922

Original commit message:
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

BUG=

Review URL: https://codereview.chromium.org/780303003

Cr-Commit-Position: refs/heads/master@{#25692}