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This allows customizing the prototype of the constructed object.

Change-Id: Id302d6f3202468963f6659e6df64726bb6bdc9d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2119183
Commit-Queue: Jeremy Apthorp <jeremya@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753347}