This is a reland of 77b24c06

Original change's description:
> [class] Ship private methods and accessors
> 
> Turns --harmony-private-methods on by default.
> 
> Intent to ship: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/HLpO-HC8aKc/TT32fUNPBAAJ
> ChromeStatus page: https://www.chromestatus.com/feature/5700509656678400
> 
> Bug: v8:8330
> Change-Id: I0d9b21cddd5418be8e7059c24fa05ab98de667b9
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2110890
> Reviewed-by: Shu-yu Guo <syg@chromium.org>
> Commit-Queue: Joyee Cheung <joyee@igalia.com>
> Cr-Commit-Position: refs/heads/master@{#66933}

Bug: v8:8330
Change-Id: I165e54982b642757b2e9ed9eb42a051ea928703b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2130451Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/master@{#67002}