Note that top-level await is already on-by-default in blink. This flips
the flag in V8 only for other embedders.

Bug: v8:9344
Change-Id: Ic860b22316718b353a0493799fdf95200a71acc1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2746843
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73416}