Add a V8 API to update load start time for RAIL PERFORMANCE_LOAD mode

Bug: v8:10947

Change-Id: If86a815f8974b47a6f4ba82c3661c21b73f39553
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2423403
Auto-Submit: Hong Zheng <hong.zheng@intel.com>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73393}