Ensure that v8 arrays are always converted to object vars when allowed.

Ensure that v8 arrays are always converted to object vars when allowed.

To maintain synchronous scripting backward compatibility, v8 arrays should
always be converted to object vars when allowed. I have added a comment
explaining the conversion of objects in more detail.

This CL also adds a check to ensure that when we convert an object var to
a v8 value that it is never nested inside of an array or dictionary. Before
we switched from NPObject to gin, this was never possible so there is no
need to add this functionality now and we should restrict this case from
ever happening.

BUG=411445

Review URL: https://codereview.chromium.org/566463002

Cr-Commit-Position: refs/heads/master@{#294785}