R=jochen@chromium.org
TEST=test-serialize/CustomContext{Des,S}erialization

Also test by
- make ia32.debug embedscript=<full path to mjsunit.js>
- d8 -e "assertDoesNotThrow('print(1)')"

Review URL: https://codereview.chromium.org/846023002

Cr-Commit-Position: refs/heads/master@{#26035}