Creates a new initializer function to instantiate instance class
fields in a base class.

An initializer function (similar to the one created for static fields)
is created during class declaration and assigned to a synthetic
context allocated variable.

This function is loaded from the variable during instantiation (when
the constructor is run) and run.

Bug: v8:5367
Change-Id: Ie11c2183b3001234ae41d7bcc2cb9b02c0764ab5
Reviewed-on: https://chromium-review.googlesource.com/754445
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49207}