Aside from the default snapshot, there is no need for additional context
snapshots to have the ability to replace the global proxy and global object
after deserialization. Changes include:
 - Changes to the API to better distinguish default context snapshot from
   additional context snapshots.
 - Disallow global handles when creating snapshots.
 - Allow extensions when creating snapshots.

This solves the issue of not being able to having accessors and interceptors on
the global object of contexts to be serialized.

R=jochen@chromium.org, peria@chromium.org
BUG=chromium:617892

Review-Url: https://codereview.chromium.org/2557743003
Cr-Commit-Position: refs/heads/master@{#41588}