GN: Enable embedder to decide whether or not the external V8 snapshot is enabled.
GN: Enable embedder to decide whether or not the external V8 snapshot is enabled.

BUG=421063

Review URL: https://codereview.chromium.org/749213004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25691}

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