Add performance tests for String.prototype.startsWith ahead of port to
torque in order to keep track of how performance is affected.

Bug: v8:8400
Change-Id: Ifc753a6f13da20c1760e545a99fd693717e3acc6
Reviewed-on: https://chromium-review.googlesource.com/c/1402934
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58685}