Allow eval in detached contexts

BUG=

Review URL: https://codereview.chromium.org/781953002

Cr-Commit-Position: refs/heads/master@{#25686}
Fix %GetWeakSetValues to work correctly with GC.

This fixes mjsunit/es6/mirror-collections failure in arm64 gc-stress bot.

BUG=

Review URL: https://codereview.chromium.org/783673002

Cr-Commit-Position: refs/heads/master@{#25687}