Add benchmarks for String.prototype.startsWith in order to keep track
of performance ahead of the torque port.

Bug: v8:8400
Change-Id: I0276b84b315024bba1a0d6f761ee2c6cf2c516a2
Reviewed-on: https://chromium-review.googlesource.com/c/1416070
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58870}