This flag was shipped on in 52, so it's due for removal. The patch includes
removing the deprecated and unused-in-Blink API Promise::Chain, and many
test updates.

R=adamk@chromium.org
BUG=v8:4633

Review-Url: https://codereview.chromium.org/2267033002
Cr-Commit-Position: refs/heads/master@{#38804}