This is a reland of bcb8d49b

TBR=petermarshall@chromium.org

Original change's description:
> [snapshot] add checksum to startup snapshot
>
> 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/1241874
> Reviewed-by: Peter Marshall <petermarshall@chromium.org>
> Commit-Queue: Yang Guo <yangguo@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#56217}

Bug: chromium:881417
Change-Id: I037f378fc2d45c3e0fa670bf538df68cbba5c53c
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/1243191Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56224}