Embed a hash of the embedded blob at serialization-time, and verify it
when loading the blob.

Drive-by: Keep the size of the blob trimmed, and only page-align for
dynamic page allocation and freeing during mksnapshot.

Bug: v8:6666
Change-Id: I10b6064f4f2847d4cc400f6f1bd6961194985b8f
Reviewed-on: https://chromium-review.googlesource.com/1019144
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52707}