Fix an error overwriting the `prototype` property of async generator
functions when FLAG_enable_slow_asserts is enabled.

Previously, the `initial_async_generator_prototype` field was never
written to the native context, and was always undefined. This caused
some incorrect runtime behaviour, and would crash when loading the field
using the Context::initial_aysnc_generator_prototype accessor when
attempting to cast the Undefined oddball to a JSObject.

BUG=chromium:771470, v8:5855
R=adamk@chromium.org, verwaest@chromium.org, gsathya@chromium.org

Change-Id: I13f2a518c59852bc77c2de1f2468a4eea457609e
Reviewed-on: https://chromium-review.googlesource.com/700261Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#48291}