benchmark: replace deprecated and eliminate var in buffer-from.js

[gengjiawen]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[Trott]

A possible problem with this change: Benchmarks are used to compare multiple versions of node to track performance. Since let in a for loop incurs a performance penalty in Node.js 6 and earlier, this will artificially show a performance increase in more recent versions of Node.js that is actually a performance increase in the use of let in a for loop and not a performance increase due to improvements in Buffer.from().

[Trott]

A possible problem with this change: Benchmarks are used to compare multiple versions of node to track performance. Since let in a for loop incurs a performance penalty in Node.js 6 and earlier, this will artificially show a performance increase in more recent versions of Node.js that is actually a performance increase in the use of let in a for loop and not a performance increase due to improvements in Buffer.from().

(On the other hand, I don't know that anyone is actually benchmarking 6 much these days. Although it does continue to show up in the charts at https://benchmarking.nodejs.org/ so maybe?)

[Trott]

^^^^^ Not blocking from me, but something for reviewers to consider.

[danbev]

CI: https://ci.nodejs.org/job/node-test-pull-request/21449/

[danbev]

Re-run of failing node-test-commit-arm (✔️)

[danbev]

Landed in 11f8024.