deps: V8: backport 61f4c22

The differences to the original patch are the replacement of
`i::IsIdentifier...()` with `unicode_cache_.IsIdentifier...()`,
because the former is not available on Node.js v11.x, as well
as the omitted `no_gc` argument for `GetFlatContent()`.

Original commit message:

    Assume flat string when checking CompileFunctionInContext arguments.

    R=jkummerow@chromium.org

    Change-Id: I54c6137a3c6e14d4102188f154aa7216e7414dbc
    Reviewed-on: https://chromium-review.googlesource.com/c/1388533


Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
    Commit-Queue: Yang Guo <yangguo@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#58562}

Refs: https://github.com/v8/v8/commit/61f4c2251e107aebca620054701f2faec36209a8
Fixes: https://github.com/nodejs/node/issues/27256

PR-URL: https://github.com/nodejs/node/pull/27259


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>