V8 can bundle user scripts in the start up snapshot. These are
shared across contexts, and do not work well context groups.

R=kozyatinskiy@chromium.org
BUG=v8:6274

Review-Url: https://codereview.chromium.org/2836623002
Cr-Original-Commit-Position: refs/heads/master@{#44847}
Committed: https://chromium.googlesource.com/v8/v8/+/9685cfd310a51b2b32f97223069abaaca77405a8
Review-Url: https://codereview.chromium.org/2836623002
Cr-Commit-Position: refs/heads/master@{#44897}