Addresses TODO by Dan --- simply by moving the check and exception
earlier in the function, before calling NewPromiseCapability() or
loading the constructor.

BUG=v8:4633
LOG=N
R=adamk@chromium.org, littledan@chromium.org, cbruni@chromium.org

Fixes 'test262/built-ins/Promise/prototype/then/context-check-on-entry'

Review URL: https://codereview.chromium.org/1561193002

Cr-Commit-Position: refs/heads/master@{#33137}