We already had checksumming for code cache data. We now extend
checksumming to the startup snapshot to catch data corruption early.

The performance impact for deserialization is a regression of 1-2%,
which should be acceptable.

Sample output for the included test with --profile-deserialization:

[Verifying snapshot checksum took 0.023 ms]
[Deserializing isolate (134348 bytes) took 1.891 ms]
[Verifying snapshot checksum took 0.024 ms]
[Deserializing isolate (134348 bytes) took 1.654 ms]
[Deserializing context #0 (47208 bytes) took 0.331 ms]
Deserialization will reserve:
    208168 bytes per isolate
    123368 bytes per context #0
Snapshot blob consists of:
    134492 bytes in 6 chunks for startup
    115272 bytes for builtins
     47152 bytes in 31 chunks for context #0
[Verifying snapshot checksum took 0.048 ms]
[Verifying snapshot checksum took 0.043 ms]

R=peria@chromium.org, petermarshall@chromium.org

Bug: chromium:881417
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ibc57520d459c86be8972f731aa35045b5e3751d7
Reviewed-on: https://chromium-review.googlesource.com/1241874Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56217}