Don't perform marking barrier for weak cell values.
Don't perform marking barrier for weak cell values.

BUG=

Review URL: https://codereview.chromium.org/1124583003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#28196}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh