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This is a reland of b3a546fe

It turned out that it's not going well to output v8_example.cc
under bindings/scripts/bind_gen/, so reverted the path to
out/Default/gen/... as same as before.

Added some asserts in example.py.

Original change's description:
> bind-gen: Support generation of function definitions.
>
> This is just a first step to support the code generation
> of bindings callback functions.
>
> Introduces CodeGenerationContext for convenience, and
> also demonstrates make_common_local_vars().
>
> Bug: 839389
> Change-Id: I9ff49b4bfc6bd98e6b29687cc01f986c2baa01fa
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1855325
> Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
> Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#709389}

Bug: 839389
Change-Id: I57ff27abdf2a01a9cc926bbf27d1a0c8ceb5ce88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880991
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709442}