Enable --harmony-object-from-entries by default.

Object.fromEntries is a new standard library method, whose proposal
(https://tc39.github.io/proposal-object-from-entries/) is currently
at stage 3.

It simply creates a JSObject from an iterable collection of key/value
pairs, such that `Object.fromEntries([ [ "a", "b" ] ]) -> { a: "b" }`

BUG=v8:8021
R=gsathya@chromium.org, adamk@chromium.org

Change-Id: Ib55bb6ca43727d66b471c0fd14845735c1ca2894
Reviewed-on: https://chromium-review.googlesource.com/c/1373918
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58262}