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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: I24fa46c4bf50526422e2f4e5e34eeb62999b9958
Reviewed-on: https://chromium-review.googlesource.com/994274Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548804}