The assertion was intended to verify that the function is only called
at appropriate times (at a point when it was called both by by other builtins, and by desugarings added in the parser and during bytecode generation) --- However, it didn't account for the case where
the wrapper Promise is resolved with another JSPromise with a
non-callable "then" method. (Step 12 of
https://tc39.github.io/ecma262/#sec-promise-resolve-functions):

"If IsCallable(thenAction) is false, then
    Return FulfillPromise(promise, resolution)."

It would be observable to verify this behaviour by loading the "then"
value and asserting that it's non-callable, so instead the CSA_ASSERT
is just removed and replaced with a comment explaining the appropriate
use of the function.

BUG=chromium:897436, v8:5855
R=bmeurer@chromium.org

Change-Id: Ib4b11abfe3339409b57ccfda9c3f75a34e0db532
Reviewed-on: https://chromium-review.googlesource.com/c/1296909
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56916}