Replace first page size in the snapshots with a heap logic that trims pages
after deserialization. The snapshot provided page sizes was just an
approximation, while the heap knows exactly where to trim.

Furthermore, trim the pages directly after deserialization, leaving no wiggle
room for further objects. This avoids pollution of the immortal immovable pages
with regular objects, e.g. Contexts. The downside is that we potentially require
expanding the space with a new page.

BUG=chromium:636331

Review-Url: https://codereview.chromium.org/2311963002
Cr-Commit-Position: refs/heads/master@{#39200}