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This change also converts the minimum necessary amount of C++ code to pass
objects of base::Value by value instead of std::unique_ptr.

Bug: 646113,799482

Change-Id: Ic8f6b1f4c120bfd7d57e045a68eb1921b8e6ab5d
Reviewed-on: https://chromium-review.googlesource.com/987238
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547966}