Stop passing raw pointers to DictionaryValue::Set, part 2

base::Value API has alternatives with unique_ptr, and passing ownership with
raw pointers can hide bugs. Therefore, this CL migrates //c/b/bookmarks and
//c/b/chromeos to use the unique_ptr-based API.

BUG=697817

Review-Url: https://codereview.chromium.org/2765363004
Cr-Commit-Position: refs/heads/master@{#459277}
24 files changed