[d8] Fix V8 external snapshot for Windows.
[d8] Fix V8 external snapshot for Windows.

Adds a cast to V8 external snapshot code for Windows.

BUG=421063, 439661
LOG=N

Review URL: https://codereview.chromium.org/842573004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26024}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh