Revert of Use same blob format for internal and external snapshots. (patchset #2 id:20001 of https://codereview.chromium.org/787033002/)

Reason for revert:
[sheriff] breaks http://build.chromium.org/p/client.v8/builders/V8-Blink%20Linux%2064%20%28dbg%29/builds/1293

Original issue's description:
> Use same blob format for internal and external snapshots.
>
> R=vogelheim@chromium.org

TBR=vogelheim@chromium.org,yangguo@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/792563002

Cr-Commit-Position: refs/heads/master@{#25729}