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/+/2110890Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/master@{#66933}