Revert of [gin] Unify snapshot loading on Windows and other platforms. (patchset #2 id:20001 of https://codereview.chromium.org/2103903002/ )

Reason for revert:
Investigate effect on Win7 startup.cold.blank_page metrics.

BUG=501799,625516

Original issue's description:
> [gin] Unify snapshot loading on Windows and other platforms.
>
> Removes extra v8 snapshot and natives validation on Windows. All platforms
> have the same validation henceforth.
>
> BUG=501799
>
> Committed: https://crrev.com/10b5a96b363a6ff23176d27a2e7fb1bba300f089
> Cr-Commit-Position: refs/heads/master@{#403430}

TBR=jochen@chromium.org,ben@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=501799

Review-Url: https://codereview.chromium.org/2141043002
Cr-Commit-Position: refs/heads/master@{#405111}
7 files changed