This fixes a race where
- mutator changes the fixed array length by trimming it,
- sweeper thread reads the length of the fixed array.

Also rename FROM_GC and FROM_MUTATOR to be more precise.

BUG=chromium:462908
LOG=NO

Review URL: https://codereview.chromium.org/1034163002

Cr-Commit-Position: refs/heads/master@{#27869}