Convert NetworkUIData to base::Value (cleanup)

This change uses base::Value instead of base::DictionaryValue in
NetworkUIData. This migrates us towards the eventualy deprecation
of DictionaryValue and will simplify some other re-factoring.

Bug: 651157
Change-Id: I2d4b9e61607b77852ece44de9990021d9b912b93
Reviewed-on: https://chromium-review.googlesource.com/1143598
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576642}
8 files changed