Fix v8 external snapshot code for Windows.
Fix v8 external snapshot code for Windows.

Needed to land https://codereview.chromium.org/826083004/.

BUG=421063, 439661
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/848443002

Patch from Ross McIlroy <rmcilroy@chromium.org>.

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