This patch adds a micro-benchmark comparing
`string.startsWith(singleCodeUnit)`, `string[0]`,
`string.endsWith(singleCodeUnit)`, and `string[string.length - 1]`.

The benchmark can be used to measure any String#{starts,ends}With
optimizations we implement in the future.

Test:

    tools/run_perf.py --binary-override-path=out/x64.release/d8 \
      --filter=JSTests/Strings/StringStartsEndsWithComparison \
      --extra-flags=--trace-turbo test/js-perf-test/JSTests.json

Bug: v8:7453
Change-Id: I68cad197fbcbfc6b1938fc437776c319ee9f81df
Reviewed-on: https://chromium-review.googlesource.com/1011619Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52589}