Partial cleanup use of base::Value in printing backend.

Bug: 660612
Change-Id: I75404c19cf794abee8eff74e78005226139682b9
Reviewed-on: https://chromium-review.googlesource.com/c/1387467
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Vladislav Kuzkokov <vkuzkokov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623194}
19 files changed