When a FunctionTemplate-based function is used as a constructor
and returns a JSProxy, we incorrectly treated that result the same
as a non-object result. Now it is treated like any other object
result, i.e., it becomes the result of the constructor call.

R=verwaest@chromium.org
BUG=v8:6294

Review-Url: https://codereview.chromium.org/2845123002
Cr-Commit-Position: refs/heads/master@{#44970}